That '70s Bell System

Begining in the mid 1960s till the very early 1980s Bell Labs intergated a Unix System into their analog telephone network. In order to make the telephone equipment preform different functions fequencies were used.

These tones draw attention to some who cared and Phreaking was born. For many this is the begining of hacking as we know it!

The Bell Phone Network — The Bell System

TWENTY SIX HUNDRED HERTZ

2600Hz is the frequency used in the old telephone network to route calls thru the system.

An idle long distance phone line was not dead or had some white-noise on it, a continuously 2600Hz tone was transmitted. To find a free route the selector would pass-thru the trunks detecting 2600Hz and size the free line.

The tone then stopped and the equipment respondent with a short burst or click sound, the

HANDSHAKE

An Operator could send 2600Hz at any time to drop the current connection and dial again without the need to size a new trunk.

DOUBLE TWENTY SIX HUNDRED HERTZ

2600Hz send twice with 30ms ON / 30ms OFF / 30ms ON would enable the operator to access special routing features like Recall.

You can hear this in the 'Pink Floyd's Collect Call to "Mrs. Floyd"' recording. Call: [ 7002 ]

TWENTY FOUR HUNDRED HERTZ

2400Hz and 2600Hz send together would make international switching equipment reset in the remote exchange. This allowed the operator to recall a number keeping the international trunk open.

MF TONES

Multi frequency tones (MF) were used to signal the desired number and establish the connection. Each MF tone representing a command signaling two frequencies at once. Like Touch Tone just different frequencies.

[KP] stands for 'Key Pulse' and is the dialing sequence entry command.

[ST] stands for 'StarT' to begin the dialing process.

Please see Wiki 'Blue Box' https://en.wikipedia.org/wiki/Blue_box & 'MF' https://en.wikipedia.org/wiki/Multi-frequency_signaling

Operator MF tones in Hz
	700	900	1100	1300	1500	1700
1	X	X
2	X		X
3		X	X
4	X			X
5		X		X
6			X	X
7	X				X
8		X			X
9			X		X
0				X	X
ST3	X					X
ST2		X				X
KP			X			X
KP2				X		X
ST					X	X

Multi Frequency Tones

THE OPERATOR

In the 1970s many long distance calls were handled by an Operator manually, operating some kind of switching equipment. You could always dial zero for an operator and get connected.

The operator handled all sorts of different calls, like Person-to-Person Calls, Collect Calls, 3rd Party Billing, or an emergency call break-through.

1970s Operator Routing Codes

Besides the Area Codes known to public the operator had special Routing Codes that could not be dialed from a local exchange and only worked in the 2600 controled long distance network.

Example of Codes

914+151 Overseas incoming (White Plains, NY)
212+151 Overseas incoming (New York, NY)
NPA+161 trouble reporting operator (defunct)
914+182 International Sender (White Plains, NY)
212+183 International Sender (New York, NY)
412+184 International Sender (Pittsburgh, PA)
407+185 International Sender (Orlando, FL)

WIKI https://en.wikipedia.org/wiki/Blue_box#Special_codes

THE INTERCEPT OPERATOR

Calls that were miss routed would go to a centralized Intercept Operator (IO).

This could be caused by the phone equipment malfunctioning or by a wrong number dialed, been changed, or not jet connected. The IO would ask "What Number are you calling please?", since there was no way of knowing the number not being transmitted by the network.

Then, calling this number again monitoring the line, giving you information what to do and punching an error report card if needed.

THE AUTOMATIC INTERCEPT SYSTEM

As call traffic rises, and time is money on the network, Bell Labs worked on a way to automate interception:

"The Automatic Intercept System routes calls to nonworking telephone numbers to a centralized location where a time division network under stored program control connects the calling customer to a series of prerecorded announcements. The customer is told what number was reached and, from information stored in a large‐capacity disc file, is given the reason the number is not in service and, if available, the new number at which the called party may be reached."
https://onlinelibrary.wiley.com/doi/abs/10.1002/j.1538-7305.1974.tb02717.x

Wiki https://en.wikipedia.org/wiki/Intercept_message

This was done by storing 96 prerecorded phrases like "The number you have reached", "2", "6", "hundred", "in the", "R", "C", "3", "Area", "has been", "temporarly", "disconncected", onto a electromagnetically rotating drum.

Saving these tracks on a computers hard-drive would have been way to expensive at the time, if not impossible considering the speech quality expected.

Each phrase is either 500ms or 1500ms long and in order to make the spoken text sound natural the spin of the drum had to be very precise. A UNIX system made sure the next track is picked on time to complete the spoken sentence.

Call [7111] to hear the number '1' being repeated over and over again as the drum would be rotating.

This would go on for decades, 120 times a minute. As the machines became old more more cross-talk of the other tracks being spoken could be heard.

Call the [8988] announcement and you can hear the track 'Call for being taken by' repeating in the background as Cross Talk.

The UNIX system would manage the numbers, status, and what track in what order to play-back to the caller. The system included an IBM 2910 Automatic Intercept Switch, IBM 7770 Audio Response Units, and an IBM 1440 Data Processing System serving some 600,000 telephones.

For each number three tracks were provided: neutral(n), emphasized(e), and lowered(l).

Depending on the position of the number in the block spoken and if it is repeating the number UNIX picked automatically the appropriate phrase. That and the timing lets it sound so natural:

0n 1e 0n 0l, or 0n 0n 1e 0l, or 0n 0n 0n 1l, or 1n 0e 0e 1l

0n 0e 0l

Call [2368] [4594] [8988] to hear the tracks in order.

AIS 96 Tracks Drum — The Automatic Intercept System

AIS diadram — The Automatic Intercept System

THAT '70s VOICE OF AIS

The voice you hear in the '2600 R C 3' track is of Jane Barbe https://en.wikipedia.org/wiki/Jane_Barbe. She must have recorded thousands of tracks and the letters R, C, and the number 3 are one of them.

Till the end of the 1970s U.S. phone numbers used 2 letters at the beginning of a number to identify the exchange by name:

PE 6-5000 is most know from the song "PEnnsylvania 6-5000" (or 736-5000). So RC 3-2600 is a correctly formatted prefix and number from that time! Thou I can't think of an exchange name beginning with 'Rc...'.

Instead of saying Zero for the last number on the dial the letter 'o' was used instead.

So the number 7001 was spoken '7 o o 1', 2600 as '2 6 hundred', and 5000 as '5 thousand'.

Bell never assigned numbers ending in 0000, but with the help of an Intercept Operator you could make the AIS say 'o thousand'.

Since 'o' is also a letter on the dial some people miss dialed 6 instead. Also callers mixed up the letter 'i' with the number 1. And this would be just another call to the Intercept Operator.

If the caller would try to trick the operator by giving a false number and for this number no error record was found in the database the AIS would announce:

"The number you have reached, R, C, 3l, -, 2n, 6e, hundred, is a working number, will you dial it again please."

THAT '70s TONES

DIAL TONE

600Hz modulated at 120Hz, continuously, 30sec time-out

To get a 1970s Dial Tone, dial:

7 0 1 0 + _📞^〜 Wait for Dial Tone, and dial Routing Code + Phone Number

You can now call the numbers listed in the 1970s directory, or let the dial tone time-out dumping you on a continuously:

Receiver Off-Hook Tone

At the end of a call when the calling party hangs-up you will be disconnected and you also get a 1970s Dial Tone.

Watch Tom Scott on Youtube: 'In Old Movies, Why The Dial Tone After Someone Hangs Up?'
https://www.youtube.com/watch?v=bUIiUXvnkUQ

What's not mentioned in the clip is the Disconnect Signal send by the phone company when the calling party hangs up. A power interruption of the phone line of 400-1000ms to release any equipment. The 1A2 Key System will release a call on hold receiving the disconnect signal.

RINGING TONE

When the bell of the phone began to ring the calling party gets a ring-back tone. The cycle you heard is the actual phone bell ringing. Party Lines used different cycles.