Tone detection system for automatic telephone exchange - recognises international three pip tone signal and stores locally dialled number

摘要

When a subscriber dials an international call the local exchange stores the dialled digits until it receives the international second dialling tone. The necessary units for detecting this tone are accordingly set up. Instead of the second diallingtone, a three pip ascending tone signal may be received indicating that international direct dialling facilities are not available. This tone signal is recognised by detecting at least two successive leading edges within a period of 1100mS. When this occurs the tone detection units are released and made available to other subscribers.