Signals and Circuit Conditions of USA
and Canadian Telephone Systems

1. The status of a local telephone line (idle or busy) is indicated by on-hook or off-hook signals as follows:

On-Hook Minimum dc resistance between tip and ring conductors of 30,000 Ohms.
Off-Hook Maximum dc resistance between tip and ring conductors of 200 Ohms.

Telephone sets give an off-hook condition at all times from the answer or origination of a call to its completion. The only exception to this is during dial pulsing of rotary or pulse dialing phones.

2. Dial pulses consist of momentary opens in the loop; dial pulses should meet the following standards:

Pulse rate: 10 pulses/second +/- 10%
Pulse shape: 58% to 64% break (open)
Inter-digital time: 600 milliseconds minimum

NOTE: Two pulses indicate the digit "2", three pulses indicate the digit "3", and so on up to ten pulses indicating the digit "0".

3. Audible tones are used in the telephone system to indicate the progress or disposition of a call. Precise dial tone consists of Current day "precise" tones consist of a summation of two low distortion sine waves. Earlier tones included below consisted of a higher frequency amplitude modulated by a lower frequency.These include (NOTE: sound files provided in two popular formats):

a. Dial tone (Real Audio) / Dial tone (WAV):  Precise dial tone consists of 350 and 440 Hz @ -13 dBm0 per tone, at telephone exchange (continuous). Earlier modulated dial tone consisted of 600 Hz amplitude modulated by 120 Hz. For Touch-Tone compatibility reasons this was replaced with precise dial tone on many electro-mechanical exchanges when they were converted for Touch-Tone calling.

b. Busy tone: "Precise" busy signal (Real Audio) / "Precise" busy signal (WAV): 480 and 620 Hz @ -24 dBm0 per tone, at telephone exchange, interrupted at 60 interruptions per minute (0.5 sec. on, 0.5 sec. off). Compare with this 600/120 Hz Modulated Busy signal (Real Audio) / Modulated Busy signal (WAV) - (Courtesy of Stephen (Steph) Kerman.)

c. Reorder (Real Audio) / Reorder (WAV): (today's standard for "all trunks busy") 480 and 620 Hz interrupted at 120 interruptions per minute. Compare with the following recording of a reorder signal (Real Audio) / recording of a reorder signal (WAV) which is a modulated 600 Hz and 120 Hz reorder tone used prior to today's electronic systems. (Courtesy of Stephen (Steph) Kerman.)

d. Ringback: "Precise" Ring-Back Tone  (Real Audio) / "Precise" Ring-Back Tone (WAV): 440 and 480 Hz @ -19 dBm0 per tone, at telephone exchange (2 seconds on, 4 seconds off). Compare this with 420/40 Hz Modulated Ring-Back Tone  (Real Audio) / Modulated Ring-Back Tone  (WAV) standard for larger metropolitan central offices of various types (courtesy Stephen (Steph) Kerman) and the Small Crossbar office ringback tone  (Real Audio) / Small Crossbar office ringback tone (WAV) standard 500/40Hz ringback tone used mostly on small #5XB but also on some SXS offices.

e. Call waiting (Real Audio) / Call waiting (WAV): 440 Hz @ -13 dBm0, at telephone exchange (0.3 sec. on every 10 seconds)

The following sound files (available in two formats) were sent from Phil P.:

OTHER TONES USED BY "MA BELL":

2600Hz single frequency tone (Real Audio) / 2600Hz single frequency tone (WAV) and 3700Hz single frequency tone (Real Audio) / 3700Hz single frequency tone (WAV) - Phone Phreaks know about these special tones! Various tones generated by blue boxes, red boxes, etc. would allow long distance calls to be made free of cost to the phreaker (but not free to "Ma Bell"!), usually from pay phones, and would allow access to central office equipment for hacking purposes. This information is presented here only as an example of other control signals used by the phone companies for internal use. An EXCELLENT web site for hearing recordings of the early cross-bar and ESS systems and other recordings made by phone phreaks during the late 1960's and early 1970's is Phone Trips, located at http://www.wideweb.com/phonetrips/.

SIT: The 3-pitch cadence tone is called SIT (special information tone), an ITU standard which preceeds all announcements. It was long used in Europe and was necessitated in the United States to permit COCOTs to distinguish between successful calls and calls which reached a recorded announcement. This was required since COCOTs did not originally receive answer supervision from the called end. They needed this to determine whether to collect or return the deposit.

Examples of SIT cadences:

• We're sorry . . . (Real Audio) / We're sorry . . . (WAV)
  

• Intercept by Automatic Intercept System (AIS) of disconnected # (Real Audio) / Intercept by Automatic Intercept System (AIS) of disconnected # (WAV) at Flushing, Queens, N.Y. the last week before the 5XB was killed in 1992. During the initial "clicks, silence, click", the local 5XB MF outpulsed the called # to the AIS center. The AIS is a time-division switch with a 96-track magnetic drum and a time slot "bus" for each announcement track. It looks up the numbers received on a multitude of trunks in a database and dynamically composes an intercept message by simultaneously sequencing each incoming trunk among the time slots. (This WAV file is 524KB so may take a while to download) - Courtesy of Stephen (Steph) Kerman.
   

• In the USA and probably other countries, dialing 911 connects you to an emergency call center. Well, listen to this actual recording someone reached when they dialed 911! Click here (Real Audio) / here (WAV) to listen. I'd hate to be the one that reached that recording as my house was burning down!
   

• Another example of a SIT tone:  To get telemarketers to quit calling your number by making their computer dialing equipment think it has reached a disconnected telephone number, play this with your answering machine at home! Using my telephone line simulator and two telephones with my Telemarketer Stopper, I recorded the SIT tones which are generated as soon as the called party picks up the phone's handset or when an answering machine picks up a call.  To hear my recording, click here (sorry, the recorded volume is lower than it should have been.)

Each of the three tones in the cadence can be one of 3 specific but similar frequencies, so the cadence will always sound the same to the human ear. The specific frequency combination used on a given call permits machine recognition of call disposition. The 3-tone SIT sequences used in the US are a subset of the 32 SIT sequences defined by the ITU. Eight of the thirty-two defined sequences are used in the US. The first tone is either 913.8 or 985.2 Hz, the second 1370.6 or 1428.5 and the third always 1776.7 (July 1776....very patriotic of AT&T wouldn't you say?). The first two tones may persist for 274 or 380 mS while the third is always 380 mS. The particular choice of frequencies and durations used on a given call is used to indicate why the call did not complete.

Alert Signal: If you wait too long before dialing, a loud, pulsating alert signal (Real Audio) / alert signal (WAV), called ROH (receiver off-hook), is sent to your phone which can be heard many feet away! This is a summation of 1400, 2060, 2450 and 2600 Hz applied at 5 PPS with a 50% duty cycle, applied at a level of approximately +5 VU measured at the MDF leaving the CO towards the customer.

The following are the signals used in the days of mechanical step-by-step switching offices:

These were used to signal dial tone, busy signals, no such number, etc. A big thanks to Paul Wills for these sound file contributions. Paul has a real 100 line Automatic Electric Step by Step Switch built in 1926 that serves his household. The ringback and busy tone are original with the PABX. (The early AE switches had no dial tone. If you picked up and heard nothing, you dialed. If you heard busy, that meant that all talking paths were in use.) The ringback is just the coupled harmonics of the vibrator generated 20 Hz. The "dying mosquito" busy tone is generated with a self interrupted relay. To see photos of some of Paul's equipment, click here.

• The "No Such Number" (Real Audio) / "No Such Number" (WAV) tone was used on unused selector levels of the AE Step by Step Switch, although this was not a normal occurrence. It was primarily used with crossbar exchanges although Paul did see a reference to connections to a manual toll switchboard.
  
• The "Busy Tone" (Real Audio) / "Busy Tone" (WAV) as generated by the AE Step by Step Switch.
  
  
• The "Ring-Back Tone" (Real Audio) / "Ring-Back Tone" (WAV) as generated by the AE Step by Step Switch (20 Hz along with all of the noise and a little bleeding of the busy tone buzz too).
  
  
• The "Dial Tone" (Real Audio) / "Dial Tone" (WAV) as generated from a WECO 101A Low Tone generator.
  
  
• This sound file is a 1000/20 in band ringing signal (Real Audio) / 1000/20 in band ringing signal (WAV). The first ring (Real Audio) / first ring (WAV) on the sound file is using the electronically generated tone. It sounds nice and clean and you can hear all the clunks as the relays in the old equipment operate. The second ring (Real Audio) / second ring (WAV) on the WAV file is generated by the old tube equipment. It sounds pretty bad but it still works. It was used for manual signaling over carrier or a composite circuit. Of course direct distance dialing has done away with manual ringing on the public telephone network but such equipment still saw use on order wires and private lines until very recently. A composite circuit is an arrangement of low and high pass filters that allowed slow speed DC signaling such as Morse Code or Teletype to share a pair with voice frequency stuff. Obviously, 20 cycle ringing wouldn't work so 1000 Hz chopped at 20 Hz was chosen as something unique that wouldn't normally occur with the human voice.The composite ringing equipment is designed to receive 20 Hz ringing on the drop side and put the ringing frequency out on the line. The other end receives the signal and shoots 20 Hz ringing towards the other drop.
  
  
• Like the 1000/20 signal, 135 Hz ringing (Real Audio) / 135 Hz ringing (WAV) was used to signal over a composite circuit. The railroad industry used a lot of this equipment at one time. The signal in this recording is generated with a vibrating reed generator. (large offices used motor generator sets) Decoding is done with a tuned reed relay. Like the 1000/20 trunk equipment, this equipment replaces the standard 20 Hz ringing voltage with a 135 Hz. signal that will not interfere with the low frequency Morse code or Teletype transmissions.

  One particularly collectable telephone is the Western Electric 312 fiddleback set. The 312 telephone is designed to directly signal over a composite circuit using 135 Hz. The telephone contains a vibrator to generate the signal. Rather than a bell, the telephone uses a device called a Howler to announce a call. The Howler resembles a watchcase telephone receiver with a funnel attached to the front. The listener actually hears the tone through the Howler.