Telephone set accepting coded key - whose coding initiates frequencies for comparing with incoming tariff-signal frequency to decide users authorisation to call given area

摘要

The telephone set accepts plug-in coded keys authorising certain users to call certain areas as described by the code on the key. The signals initiated by the keys when inserted into the set are a.c. signals whose frequency is determined by the key coding. These frequencies are compared with the call-tariff signals coming from the exchange. If the clock frequency of these tariff signals exceeds the frequency of the signal initiated by an inserted key, a switch is operated that disconnects the connection. The incoming tariff signals are passed to a store and display located in the key.