Ringing (telephony)
（重定向自Ringing generator）
A ringing signal operates a bell or other ringtone generator in a telephone, to alert the user to an incoming telephone call.
In landline telephones, bells or ringtones are rung by impressing a 60 to 105-volt RMS 20-Hertz sine wave across the tip and ring conductors of the subscriber line, in series with the (typically) −48 VDC loop supply. This signal is produced by a ringing generator at the central office.