A ring tone is the sound
made by a telephone to indicate an incoming call. The
term, however, is most often used to refer to the
customisable sounds available on mobile phones. This
facility was originally provided so that people would be
able to determine when their phone was ringing when in
the company of other mobile phone owners.
A phone only rings when a special "ringing
signal" is sent to it. For regular telephones, the
ringing signal is a 90-volt 20-hertz AC wave generated
by the switch to which the telephone is connected. For
mobile phones, the ringing signal is a specific
radio-frequency signal.
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