For example, rings with a shorter interval between them might be used to signal a call from a given number.
A ringing signal is an electric telephony signal that causes a telephone to alert the user to an incoming call.
For mobile phones, the network sends a message to the device, indicating the incoming call.
The caller is informed about the progress of the call by the audible ringing signal, often called ringback tone.
This is done at the Central Office, or a neighborhood multiplexer called a "SLC" for Subscriber Line Carrier.
In POTS switching systems, ringing is said to be "tripped" when the impedance of the line reduces to about 600 ohms when the telephone handset is lifted off the switch-hook.
This signals that the telephone call has been answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call.
Arguably the first ringtone (in the modern sense) appeared in the movie Our Man Flint in 1966, where the head of the secret government agency had a red phone that went directly to the President and rang with a distinctive musical ringtone (probably made by the sound effects crew using an early analog synthesizer).
Following a 1975 FCC ruling which permitted third-party devices to be connected to phone lines, manufacturers began to produce accessory telephone ringers which rang with electronic tones or melodies rather than mechanically.
For landline telephones typically receive an electrical alternating current signal, called power ringing, generated by the telephone exchange to which the telephone is connected.