this circuit provides secrecy when two or more telephones are connected in parallel to a telephone line. the circuit also prevents incoming calls to as well as outgoing calls from other telephones connected in parallel, except from the one lifted first.
When someone picks up the handset of the telephone connected to the original (master) phone for making an outgoing call, no dial tone is heard and the phone appears to be dead. but when call comes, the ring signal switches the SCR's 'on' and conversation can be carries out. As soon as the handset is kept on the hook, the SCR goes off and the telephone can again only receive incoming calls
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