Telephone Signaling System Technologies Past & Present


Signaling is a very important part of a telephone call. Without signaling, a call cannot be sent through the telephone network from one end to the other. Signaling methods were developed to properly send information on who the caller wants to call (the called party) and who the caller is to the network (identification for billing). The following are examples of various technologies, both past and present that were or are still used within the North American telephone network.

Telephone Signaling System Technologies

A brief overview of the history and operation of telephone signaling technologies.


Pulses (interrupting current on toll trunk lines), especially on inter-office trunk lines, were common in the days of step by step and crossbar systems. These were used to pulse out dialed digits, either directly or via pulse senders from a central office switch, to the destination switch. Though not in common use today, these may still do exist in rare instances outside of North America.

DC Voltage and DC Polarity

Voltage changes and polarity changes (as well as pulses) were used in some inter-office signaling routines. These were sometimes found in Panel systems when communicating to Panel “tandem” systems. Never was in common use and are not used anymore.

Multi-Frequency Signaling

MF (Multi-Frequency) signaling was developed in the 1930s for signaling between central office and toll tandem systems. Special dual frequency tones (not the same as Touch Tones) are used to send information of the called telephone number throughout the network. These tones have to be within the band of normal voice communications in the telephone system (300Hz to 3000Hz). The information is sent to either tandems and then to local central offices, or from central office to other central offices directly. The format of a typical MF sequence on most calls is:

KP+dialed digits+ST

where KP = Key Pulse and ST = Start. Most MF toll trunks in the US and Canada use 2600 Hz as supervision and for idle line. Some places used 3750 Hz (out of band) for supervision/idle line control.

An abbreviated listing of MF tones in the North American network (sometimes referred to as R1 signaling) are shown in the chart below. KP2 is not normally used in domestic traffic. 

Digit Frequency #1 (Hz) Frequency #2 (Hz)
KP 1100 1700
KP2 1300 1700
1 700 900
2 700 1100
3 900 1100
4 700 1300
5 900 1300
6 1100 1300
7 700 1500
8 900 1500
9 1100 1500
10 1300 1500
ST 1500 1700



MF signaling was used until the early 2000s in limited amounts in the US and Canada. An international version of MF signaling (sometimes referred to as R2 for bi-directional signaling) is still in widespread use throughout the world.

Common Channel Inter-Office Signaling / Signaling System 7

CCIS (Common Channel Inter-Office Signaling) started in the late 1970s as a way to send more information between tandems and switches in the network. Sending a call via MF takes up to 15 seconds on a domestic phone call. Also, MF lead itself to toll fraud since making MF tones is fairly easy to do.

CCIS was invented to send information “out of band” on a data circuit parallel to the voice circuit. This way the call can be set up and completed in a shorter amount of time, be able to send more information, and avoid toll fraud – all at the same time.

The early commonly deployed version of CCIS was version 6. The modern version of CCIS is version 7 – commonly called SS7 (Signaling System 7) where there are 7 “layers” of the signaling network. SS7 is in widespread use in the USA, Canada and many industrialized countries in the world including the UK, Australia and others.

Other Telephone Historical Pages

Landline Telephone History – Main Page
Main page for Telephone History at Telephone World

A Visit to the Connections Museum Seattle
Pictures taken at the Museum of Communications in Seattle, Washington in May 2005.

AT&T Crash of 1990
An excerpt from a magazine with details of the “crash” of the AT&T long distance network in January 1990.

Morris, IL Electronic Switch Experiment
A technical history for the first “electronic” switching system field trial experiment in Morris, IL from 1958 to 1962.

History of the (former) North Pittsburgh Telephone Company
An unofficial technological history of the former North Pittsburgh Telephone Company in Gibsonia, PA (suburban Pittsburgh) from its origins to when it ceased to exist.

Telephone Signaling System Technologies Past & Present
Details on the signaling methods used within the North American telephone network to allow central office and tandem systems to “talk” with each other.

Telephone Transmission Technologies Past & Present
Details on the physical means on how a telephone call is handled through the network between different systems.

Old Bell System Operator Routing Codes
A historical listing of old routing codes that Bell System (AT&T Long Lines) operators used to use that performed special functions within the toll network.

Old Number 4 Crossbar Toll Tandem List
An extensive historical listing of Bell System Number 4 Crossbar (#4XB) toll tandem systems from the 1940s to the 1980s.

Other Telephone History
Technological history on various telephone company systems from the webmaster’s personal perspective over the last 40 years.

Historical Videos
Various videos from YouTube that we’ve found that showcase telephone history.