| 摘要 |
To prevent unauthorized use of a telephone, a 4-digit access code must be dialled before the desired number. The dialled access code digits are detected (at 21, 22, 23) and each digit in turn is compared (at 26) with a corresponding stored access code digit from memory (25). A line reset relay (13) is opened on each such digit, so signalling a dialling abort and hang-up to the exchange. If the dialled and stored access codes match, flip-flop (35) remains set, and after the 4th digit gate (36) inhibits further comparisons, so the desired number can then be dialled. Otherwise, the operation repeats, with the system again monitoring for the access code. Means also allowing calls to emergency and a single programmable number can be provided. The system can also provide logging of calls from a private exchange, and control of calls depending on time, whether the call is long-distance or international, etc. |
