A ringing signal operates a bell or other ringtone generator in a telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

, to alert the user to an incoming telephone call

Telephone call

A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...

.

Telephone bells or ringtones are rung by impressing a 105-volt RMS 20-Hertz sine wave across the tip and ring

Tip and ring

"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...

 conductors of the subscriber line. This signal is produced by a Ringing Generator at the central office. When the subscriber line is called, a relay on the subscriber line card

Line card

A line card or Digital Line Card is a modular electronic circuit on a printed circuit board, the electronic circuits on the card interfacing the telecommunication lines coming from the subscribers to the rest of the telecommunications access network.A line card commonly interfaces the twisted pair...

 connects the Ringing Generator to the subscriber line. The exchange also sends a ringback tone

Ringback tone

A ringback tone is an audible indication that is heard on the telephone line by the caller while the phone they are calling is being rung. It is normally a repeated tone, designed to assure the calling party that the called party's line is ringing, although the ring-back tone may be out of sync...

 to the calling party

Calling party

The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

. When the called party

Called party

The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

 answers by taking the telephone handset

Handset

On a telephone, the handset is a device the user holds to the ear to hear the audio sound. Modern-day handsets usually contain the phone's microphone as well, but in early telephones the microphone was mounted directly on the telephone itself, which often was attached to a wall at a convenient...

 off the switchhook, the subscriber's telephone draws direct current

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

from the central office battery. This current is sensed by the line card and the ringing relay is de-energized.