This circuit generates the busy tone of ordinary telephones. The transistors can be the BC548 and the components of the T filter and the multivibrator can be changed to obtain the exact tone.
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....