Historically, this entailed sending a high-voltage alternating current over the telephone line to a customer station which contained an electromagnetic bell.
[1][2] In landline telephones, bells or ringtones are rung by impressing a 60 to 105-volt RMS 15 to 25 Hertz sine wave (depending on manufacturer and telecoms administration) across the conductor pair of the subscriber line, typically represented by tip and ring, in series with the (typically) −48 VDC loop supply.
While a telephone exchange includes a central source of ringing voltage at the switchboard, a private branch exchange or telephone-based intercom must provide a local source of AC ringing voltage.
A closed system, such as an intercom or private branch exchange, need not comply with standards for ringing if it does not use standards-compliant telephone extensions.
Some office telephones replace the bells with separately-wired low-voltage DC buzzers or use entirely custom-designed extensions which only work with one manufacturer's switchboard.
These avoid the need to generate 20 Hz 90V sinusoidal AC, but are not compatible with standard telephone ringers.