| 摘要 |
2077747 9114330 PCTABS00007 A telephone circuit is provided for distinguishing between a normal incoming telephone call and a test telephone call. A series connection of capacitor (C1), bell (2) and relay (13) is provided across the telephone line pair (1). The relay (13) is normally open so that a ringing signal appearing across the telephone line pair (1) does not cause the bell (2) to ring. The ringing signal is sensed by a ringing detector (15) which supplies an output signal to a processor (6). If the ringing signal is present for more than a predetermined time, such as three seconds, the relay (13) is caused to close, thereby enabling the bell (2) to sound. When the telephone is polled from a central station to cause it to undergo a self-test routine, a ringing signal is used which consists of a series of three ringing signals each lasting less than the predetermined time. Consequently, when such a ringing signal is received, the relay (13) remains open, and there is no audible ringing tone. The system thus enables a testing routine to be carried out by the telephone without causing the telephone to ring.
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