Internal calling between two phones on small PBX - operates after delay when one receiver lifted unless key pressed for outside call

摘要

In a small PBX with one exchange line and two phones, either can call the other by lifting the receiver and spinning the dial. The dial pulses are detected and connect a local ringer to the other phone. To make an outside call, an earth key must be pressed before dialling. However, contact bounce on the hook switch is liable to call the other phone before the E key is pressed. This defect is avoided by introducing a time delay of 4s before ringing the other and dispensing with the need to spin the dial. This is long enough to replace the receiver if the other is found to be engaged on an outside call.