Ringing and Tones

This page has a lot of technical information about the equipment that rang telephones and produced tones, such as busy signals and dial tones in the old network. There are a lot of sound samples.

If you have trouble playing back samples, it could be due to Web browser issues. It has been reported that some users of Internet Explorer have not been able to play the sound samples. Try Safari, Firefox or Chrome browsers. In a few cases, even on these browsers, you may have to reload the page to get a particular sound to play. This is something we are investigating

Early Methods of Ringing Telephones

When the ringer was first invented for telephones, a Magneto Generator was used to generate the power to ring it. The magneto was built into the wooden box type phones and had a crank on the side of the phone to operate it.


Even the central office switchboards up to about 1900 had hand cranked magnetos. The operator had to turn the crank to ring each subscriber. There was either no reliable commercial power available in the central office at the time or no method to convert the commercial power into the 17HZ to 20HZ needed to ring telephones.

With commercial electrical power, the simple solution was to have an electric motor . A motor turned the magneto generator to produce the required voltage and frequency to ring customer telephones. The Ringing Machine was born.

The Ringing Machine was fine for larger manual offices but too expensive for small single position switchboards. For those, various versions of a device called the Pole Changer were used. This was a mechanical device that would convert Direct Current from storage batteries into Alternating Current for ringing by reversing the polarity of the current at regular intervals, in a vibrating type motion. These were not particularly accurate.

With an entirely manual system, there was no need for tones. The operator reported the status of the call verbally to the caller. Tones were added when dial service started. In cities with mixed manual and dial service, even the manual offices needed tones.

Types of Tones

Ringback Tone is the name for the tone that caller hears when the called party's number is ringing.

Originally, the tone used for dial tone and busy signals was the same tone, called Low Tone. Busy tone is interrupted at 60 IPM. Fast busy tone — also known as Overflow or Reorder — is interrupted at 120 IPM. There was a slower version interrupted at 30 IPM that was used only in the toll network with Operator Toll Dialing, which mostly disappeared after the introduction of Direct Distance Dialing.

In addition to Ringback Tone and Low Tone, there was High Tone, which was usually 500HZ. This was not present in all offices. This was also called Howler tone and was originally used as the "off-hook" tone. The more modern off-hook town appeared around the 1970s.

There was also a siren tone that was used to indicate a vacant code in a dial office.

Western Electric Power Plants with Motor-Generator Machines

The following power plants had motor-generator ringing machines that produced tones for dial tone, busy tone and ringback tone, in addition to generating the 20HZ AC current necessary to ring telephones.

Audible Ringback Tone was superimposed on top of the 20HZ ringing voltage. When the called party's telephone was ringing, the caller heard the sound of the actual signal going out to the called telephone. On top of the ringback tone, the caller could hear any noise present on the called party's line.

There were two ways that tones were produced. The early equipment used a commutator. The later equipment used tone alternators.

A commutator basically interrupted the direct current at a specific rate, such as 160 interruptions per second to create a low pitched tone and 500 IPS for a high tone. These were basically square waves and not pure tones.

An alternator is basically an electrical generator, creating alternating current at a specific frequency. These were sine waves and therefore pure tones.

P-Type Ringing Machine

This equipment was introduced in the early 1900s and used on Manual Offices, Panel offices and Step-by-Step offices up until around 1931. There was a single ringing machine provided, with both an AC motor, driven from commercial power and a backup DC motor, that would be driven by central office battery, in the event of a commercial power failure.

These machines originally used commutators to produce tones. The tones had a raspy sound.

Most of the P-Type machines were modified later to add a tone alternator that produced the well known "City Ring" that was also used on the 803C machines below. This likely occurred mostly in the 1950s. In Atlanta, it was noted in the newspaper that the two Downtown Panel offices were upgraded to new tones in 1949. The East Point office and other early step-by-step offices that probably had P-Type machines were updated in the mid 1950s. Some machines, like the New York example above, were never upgraded.

803C Power Plant / Ringing Machine KS-5396

803C_power_plant.png 803C_machine_sm.jpg

This power plant was used on No. 1 Crossbar, larger No. 5 Crossbar and No. 1 Step-by-Step offices.

There were two motor-generator ringing machines — a primary and an alternate — mounted on a table. The ringing machine was developed in 1931, according to Bell Laboratories Record.

Two switching machines in the same building could share the same 803C plant as long as the total power output was enough. It was common practice for two No. 5 Crossbar machines to share one 803C plant or maybe even a Panel machine and a crossbar machine. It appears that Step-by-Step did not usually share the same 803C power plant with a Crossbar office. In the cases where Southern Bell in Atlanta installed a No. 5 Crossbar in the same building as an existing Step-by-Step machine with an 803C, they put in a separate power plant and ringing machine for the No. 5 Crossbar.

The 803C equipment continued to be installed in new No. 5 Crossbar offices to the end of the 1960s. The last one in Atlanta was the East Marietta office, installed in 1969.

City Ring

Tone alternators produced the well known "City Ring" sound, made up of 420HZ modulated by 40HZ. The two tones were mixed with the ringing current at 20HZ and tended to have other harmonics as well.

The ringback tone had a musical quality that is probably the most endearing part of the old switching network that we lost when everything became electronic and sterile in the 1980s.

Mercury Drum Interrupters

Mercury drum interrupters were used for interrupting the ringing cycle into a ringing interval and a "silent" interval. They also provided interruptions for other tones.

The interrupters created noise that often sounded like static or a "SHHHH" sound, particularly during the silent interval between rings. This sound varied between installations. It is also likely that the machines developed these noises as the machines aged. In some recordings of machines in Pacific Telephone offices in California, there was almost no extraneous noise. Perhaps Pacific Telephone performed more extensive maintenance on their equipment than telcos on the east coast.


The machine in the picture appears to have 9 drums. The machine could be ordered with different options. Each drum acted as an on/off switch with regular timed intervals.

Ringing Cycles

For ringing, two drums were required for each ringing cycle: one to turn the ringing current on and off and another to apply DC current between rings (the silent interval). During the silent interval, direct current needed to be applied to the called line so that answer would be detected if the customer picked up in between rings.

The standard ringing pattern for private lines was 2 seconds on, 4 seconds off. There would always be 3 "phases" of this, output from the ringing machine interrupters to the power distribution wiring going to the switching equipment in the building.


The 3 phases (A, B and C) were offset so that one cycle was always ringing. If many customer telephones were being rung at the same time by different circuits in the office, they were divided up so that only 1/3 of the output of the ring generator would be needed.

The above pattern was "single ring" for private line, 2-party customers and two of the parties on a 4-party line.

Some of the ringing machines had an additional set of interrupters to create "Double Ring" for 4-party and 8-party lines.


The double ring pattern may have had additional phases, like the single ring above. But there was not room in the sequence for 3 evenly spaced phases like on the single ring.

Double Ring may have been an optional feature since not every office required it. It is likely that there were no additional patterns, such as 3 rings or long-short rings on this particular machine.

In Step-by-Step offices with a full set of Double Ring interrupters, the common practice was to set up half of the telephone numbers to use Single Ring and the other half to use Double Ring. In the Atlanta 63 office, the ring was determined by the hundreds digit, where odd numbers like X1Xx would be Single Ring and even numbers like X4XX would have Double Ring. This practice was also followed in some cities with No. 1 Crossbar and probably Panel.

One more interrupter was required to provide a "Ring Start" signal for each phase of double ring. This interruptor provided a timing pulse to the switching equipment at the start of a ring cycle. The switching equipment would delay the start of ringing until the Ring Start signal to prevent a called party getting a partial cycle on the first ring.

Busy and Reorder Interrupters

Low Tone interrupters created Busy Tone and Reorder Tone at 60IPM and 120IPM, respectively. These only needed one drum each, unlike the ringing interrupters.

In Step-by-Step offices, the Low Tone was distributed from the ringing machine to the switching equipment as 3 signals: Dial Tone (uninterrupted), Busy and Reorder. Busy and reorder tones were made intentionally loud. This design caused these tones and the associated interrupter sounds to bleed through and be heard in the background in various situations. You could often hear reorder tone during the silent interval of ringing. You also could hear the interrupter clicks of the reorder tone as soft ticks during the dial tone.

Crossbar offices did not distribute busy and reorder tone the same way as Step-by-Step. It appears (to be confirmed) that only the timing pulses (60IPM and 120IPM) were distributed to the switching equipment and that relays in the equipment turned the low tone on and off as required. As a result, there was little or no bleed through of these signals into ringing or dial tones on crossbar offices with 803C equipment. It is also possible that double ring may have been handled this way in some crossbar offices.

Examples of Tones

There were many other subtle variations of sounds produced by this equipment. And there were was often crosstalk from other circuits. The Tucker 938 office had a rare defect that caused sounds from the other ringing cycles to noticeably bleed into the current ringing cycle. This was caused by something in the wiring of the power plant or the switching machine and not the ringing machine itself because the exact same thing was heard on the backup ringing machine.

Speed Variations

With these ringing machines, the Primary machine, being powered by 60HZ commercial power, used a synchronous motor and maintained a constant speed. The nominal rotation speed was supposed to be 1200RPM. The backup or "alternate" machine was powered by 48V DC current and was speed regulated by a speed governor. This meant that it could run at an inaccurate speed and was often different from the primary machine. Below is an example of the alternate machine in the Tucker office. You can contrast this to the sample above.

The primary machine had a very consistent speed but it actually ran slightly slower than 1200RPM, possibly 1120RPM. This meant that the tones we were used to hearing were lower pitched than was probably intended. The ringing frequency was also lower than the nominal 20HZ: about 18.6HZ.

804C Power Plant / Ringing Machine KS-15532

This power plant was introduced in the Bell System some time in the late 1950s or early 1960s. It was most commonly used in smaller No. 5 Crossbar offices, having a single marker group. It was also used on some No. 1 Step-by-Step and No. 350A Step-by-Step offices.

The first of these machines would have been heard in the Atlanta area in 1963 when the Marietta No. 5 Crossbar office was built, serving the "422" and "424" codes. Additional Atlanta metro offices received this type of equipment up through 1968. Crossbar installations were in Buckhead "261", Norcross "448" and Conyers "483". The equipment was also installed on a Step-by-Step machine downtown that provided Centrex for Southern Bell Telephone official use on the "529" code.

Crossbar offices in other Georgia towns also had this equipment, including Cartersville, Covington, Tifton and Millen. It was not nearly as common as the 803C machine.

There were two motor-generator ringing machines in pull-out drawers. The interruptors on these machines were based on cams rather than mercury drums. There was no "SHHHH" sound between rings, like on the 803C, but there was an annoying "crackling" sound heard on top of the ringing tone. We didn't compare the sound during the daytime to late at night to see if this was due to traffic but the one on the small Step-by-Step Centrex did not have the crackle. So that seems to support the theory.

Other than the exceptions noted below, the equipment produced an audible ringback tone that was made up of a 500HZ tone modulated by 40HZ. This was noticeably higher pitched than the city ring of the 803C machine. Since it was mostly associated with No. 5 Crossbar offices, it became known in some circles as "Crossbar Ring". The low tone was also a higher frequency than that of the 803C.

Outside of Atlanta, there were other No. 5 Crossbar machines that had a similar tone plant with the distinctive "crackle". It is believed these were also the 804C machines but had a lower pitched tone than 500HZ. They may have existed only exported or OEM versions of the No. 5 Crossbar outside of the Bell System. Below is an example in Montreal. Another one in Kingston, Jamaica had the same sound.

Unknown Ringing Machine on Early No. 5 Crossbar

Some of the early No. 5 Crossbar central offices in Long Island, New York had a very different ringback sound. There was very little "tone". It was mostly made up of harmonics of the 20HZ ringing current. It could have been some early version of the 804 machine without the 500HZ tone superimposed.

805 Type Power Plant / Ringing Machine KS-5546 and KS-5659

KS-5546B.png KS-5546A.png

(The power plant number may be incorrect)

Community Dial Offices (350A and 355A Step-by-Step) as well as larger PBXs used this machine. These ringing machines did not have a tone alternator like the 803 and 804 machines and had a rather harsh, raspy ring sound. The Low Tone, used for dial tone and busy tones was produced by a commutator, just like the ringing machines from the 1910s.

The machines had a lot of interrupters on them, to produce a greater number of ringing patterns for the many party lines that were used in rural areas. Even the standard "single ring" was a shorter interval than what would be used on a city ringing machine, like the 803 or 804 type machines above. It was typically 1 1/2 seconds, instead of 2 seconds.

These machines had frequent speed variations that appeared to be caused by the electrical load placed on the ringing generator when customers were being rung. If you were listening to dial tone or busy tone, you would frequently hear the pitch decrease and increase again based on the ringing cycles that were occurring at the same time for other customers.

All of the small Step-by-Step offices outside of Atlanta, within the local calling area, had this type of ringing equipment.

Examples of Ringback Tone

Low Tone

Speed Variations

These machines tended to have frequent speed changes, caused by heavy load on the ringing generator during busy periods. You could hear this mostly in the dial tone and busy signal.

sample needed

806G Power Plant / Ringing Machine KS-15804

This was a small motor-generator machine used on 701 and 740 PBXs. The sample below is the typical sound but there were some variations heard.


You would not have heard this machine on the local network unless you were calling into a PBX, belonging to a business. The above example comes from a PBX set up for Centrex service for Delta Airlines on the "346" prefix. You would have heard the same type of ringing when calling the downtown banks on the "588" prefix at that time.

Western Electric Plants with Static Ringing and Tone Generators

806F Power Plant / Sub Cycle Generators KS-5585, 5523, 5593 and 5756

This plant was used on smaller Step-by-Step Community Dial offices. It was probably introduced in the 1940s. It used the 60HZ commercial power line frequency to derive the Dial Tone and Busy tones. Ringing was provided by a Sub Cycle power converter that derived 20HZ ringing from the 60HZ power frequency. The Sub Cycle unit also generated the ringing tone, which could be described as a "flutter", although less polite terms have been used.

There was a standby motor generator that would be used in the event of a commercial power failure. It would run on the 48V office batteries and generate the 60HZ required for the static ringing generator and tone generator.

There were no offices in the Atlanta local calling area with this power plant and tones. The closest office with this equipment would have been Flowery Branch, Georgia.

812A or 808A

For No. 1 ESS

sample needed


No 2 ESS

852A (or 853A)

This plant used solid state components to generate the 20HZ ringing current and tones. The tones used were called "precise tones" and were the same tones associated with ESS equipment and the same tones in use in the 2000s on digital offices. The ringing tone consisted of 440HZ plus 480HZ, superimposed over the 20HZ ringing.

This equipment was used on late No. 5 Crossbar machines, starting in around 1970. In the Atlanta area, offices that had this were Dunwoody "394", Snellville "972", Lilburn "923" and Morrow "961".

There was a similar plant used on the small No. 5A Crossbar machine. The only example of that in the Atlanta metro area was South Douglasville "489".

In other cities, a version of this system was sometimes installed in Step-by-Step offices, possibly as a replacement for older power equipment.

Automatic Electric Company

Automatic Electric used Motor-Generator equipment that was similar to Western Electric equipment but they did not superimpose audible ringing tone on top of the 20HZ ringing current. They provided separate pathways through their switching equipment for the two signals. This meant that the caller heard more of a pure ringback tone that was not affected by the characteristics of the 20HZ ringing voltage on the called telephone line. This was also important because Automatic Electric used harmonic ringing on party lines. The ringing frequency used depended on the individual party and might not be 20HZ. It could be 16 1/3HZ, 50HZ or something else. If the ringback tones were mixed with the ringing current, the sound would be different for each party on a party line.

AE also tended to use shorter ringing intervals, even shorter than the Bell System CDOs.

Stromberg Carlson Company

Stromberg made a power plant with a static tone generator for the X-Y Step-by-Step system that used vacuum tubes to generate the tones. The dial tone was rather pleasant. There were a few different versions of it. The one in Homer, Georgia sounded very much like a fog horn.

The audible ring tone was rather unpleasant.

North Electric

North was a company in Galion, Ohio, that produced crossbar (NX-1, NX-2) and all-relay (CX-100, CX-1000) switching equipment for the independent telephone market. The crossbar equipment was manufactured as a licensed adaptation of equipment made by LM Ericsson of Sweden.

The North NX-1 system probably had two different tone plants. The older machines had a Low Tone and Ringback tone that was similar in sound to the Automatic Electric machines, only the ringback was a lower pitch and the rings tended to be longer (2 second on, 4 seconds off), like Western Electric city machines.

In Georgia, NX-1 machines were found in Commerce, Moultrie and possibly other towns.

The newer NX-1s had a tone plant that sounded similar to Western Electric's solid state 852A machine, having "precise" tones with the ringing tone mixed in with the 20HZ ringing current.

sample needed Commerce GA)

The North CX-100 and CX-1000 All-Relay machines used vibrating relays to produce tones.

sample needed

Other Notes

Link to a British Post Office ringing machine operating. It includes the tones.

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