HOME > Technical Information > Signal & Circuit Conditions for USA/Canada
Clark Howard's Website Link - To get telemarketers to quit calling your number by making their computer dialing equipment think it has reached a disconnected telephone number, record this special tone sequence with your answering machine just before the beginning of your message!
Or, if you would rather just buy a device that will produce these tones for you automatically, the Telemarketer Stopper sold by Mike Sandman Enterprises will do the trick. Here is a photo of the device attached to my answering machine. Similar devices are also available at local electronics stores. UPDATE: The Telemarketer Stopper has been banned from the market due to patent infringement. Other units similar to it are available but, for legal reasons, I can make no purchasing recommendations.
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.:
Real Audio Files:
|Error-You Have Reached . . .||Error-You Have Reached . . .|
|Error-Please Try Your Call Again . . .||Error-Please Try Your Call Again . . .|
|Crossbar call completion sound <== not sure about this one yet||Crossbar call completion sound <== not sure about this one yet|
|Small Crossbar office ringback tone - standard 500/40Hz ringback tone used mostly on small #5XB but also on some SXS offices.||Small Crossbar office ringback tone - standard 500/40Hz ringback tone used mostly on small #5XB but also on some SXS offices.|
|Precise dial tone from a Crossbar office - This is a dial tone connection being set up on a #5XB (probably a earlier flatspring version) followed by the dial tone.||Precise dial tone from a Crossbar office - This is a dial tone connection being set up on a #5XB (probably a earlier flatspring version) followed by the dial tone.|
|2 multi-frequency outpulsing sequences||2 multi-frequency outpulsing sequences|
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:
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.
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.