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) -48VDC 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.
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