Ringtones

A ringtone or ring tone is the sound made by a telephone to indicate an incoming call or text message. Not literally a tone nor an actual (bell-like) ring any more, the term is most often used today to refer to customizable sounds used on mobile phones.

A telephone rings when the telephone network indicates an incoming call, so that the recipient is alerted of the calling attempt. 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. 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. Power ringing and audible ringing are not synchronized.

Telephones with electromagnetic ringers are still in widespread use. The ringing signal in North America is normally specified at ca. 90 volts AC with a frequency of 20 hertz. In Europe it is around 60–90 VAC with a frequency of 25 Hz. Some non-Bell Company system party lines in the US used multiple frequencies for selective ringing.

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. 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. 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. (SLC is a trademark of Alcatel-Lucent, but is often used generically.)

...
Wikipedia