Telephone extension

In many countries, unlike most of the UK, it is still not possible to parallel two or more telephones easily. The circuit described here may either be connected in parallel to an existing telephone or be used as a stand-alone unit, when it will energize a relay on receipt of an incoming call. The relay may be used to operate an optical or an aural indicator, or cause a trigger pulse to be generated that actuatesan interface, which in turn operates a suitable telephone.