Found elsewhere in this section, but from other sources, this circuit is from Motorola documentation. The circuit uses a triac of the time and a tripping transformer. The circuit is for 110 V network.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit...
This interesting intercom circuit was obtained from an Argentine publication from the 1970s....