While the sound produced is still called a "ring", more-recently manufactured telephones electronically produce a warbling, chirping, or other sound.Variation of the ring signal can be used to indicate characteristics of incoming calls.
On a POTS interface, this signal is created by superimposing ringing voltage [90 volts AC at 20 Hz in the USA] atop the −48 VDC already on the line.
This is done at the Central Office, or a neighborhood multiplexer called a "SLC" for Subscriber Line Carrier.
In Australia and the UK, the standard ring cadence is 400 ms on, 200 ms off, 400 ms on, 2000 ms off.
These patterns may vary from region to region, and other patterns are used in different countries around the world.
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.
For landline telephones typically receive an electrical alternating current signal, called power ringing, generated by the telephone exchange to which the telephone is connected.
The ringing current originally operated an electric bell.
Some central offices offer distinctive ring to identify which of multiple numbers on the same line is being called, a pattern once widely used on party line (telephony).