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.
When the subscriber line is called, a relay on the subscriber line card connects the ringing generator to the subscriber line. The exchange also sends a ringback tone to the calling party. When the called party answers by taking the telephone handset off the switchhook, the subscriber's telephone draws direct current from the central office battery. This current is sensed by the line card and the ringing relay is de-energized.
On multi-party lines, ringers would typically be connected from one side of the two-wire line to ground; a "tip party" and "ring party" would have bells connected from opposite sides of the line. On a two-party service, each user would not hear ringing for calls to the other party. Some 20th-century independent telephone companies deployed four-party lines which used differing frequencies for selective ringing of individual parties (the four possible combinations were 20Hz or 30Hz from tip to ground, 20Hz or 30Hz from ring to ground). If additional parties were added to the same line, distinctive ringing patterns would need to be used to identify the called subscriber; these were audible to the multiple users on the line.
A ring generator or ringing voltage generator is a device which outputs 20 cycle sinusoidal AC at up to 90 volts peak to power bells or annunciators in one or more telephone extensions. The output stops if a handset is taken off the hook.
In terminology devised by phone phreaks, a ringing generator is a magenta box. The device can be used to simulate an incoming call by applying the AC ringing signal voltage to a POTS line to make the telephones on that line ring. A magenta box in conjunction with an orange box (which emulates the caller ID signal to create falsified data) is called a vermilion box.
The hardware also has multiple legitimate applications. 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. Theatrical performances often deploy a ringing telephone on-stage as a stage prop. An analog telephone adapter for voice over IP applications has to provide its own ringing voltage generator and other line signaling services for standard telephone extensions. The hardware may also be useful to technicians for test purposes.
Stand-alone ringing voltage generators are commercially manufactured; ring generators are also pre-built for inclusion in other telecommunications equipment and various circuits published by hobbyists to generate or detect ringing voltage for analogue telephony.
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 20Hz 90V sinusoidal AC, but are not compatible with standard telephone ringers.
- "AUSTRALIAN STANDARD AS/CA S003.1:2010 Requirements for Customer Access Equipment for connection to a Telecommunications Network" (PDF). 2010.