| 摘要 |
A module is connected in the line to a subscriber telephone. An incoming call does not cause the telephone to ring unless the subscriber number is followed by a three or four-digit code. This code is given only to friends and acquaintances of the subscriber. A variant of this scheme allots a discrete code to each prospective caller. Then, if a code is compromised, only that one code need be deleted from module memory instead of having to circulate a new code to all prospective callers.
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