In the figure we have one of the many options found in this section for an intercom using ordinary phones not in use. This was obtained in a Popular Electronics from the 90s.
This circuit is recommended for alarm systems since it has a good output stage. The circuit is...
) Suggested by Philips in a 1968 documentation this circuit has very high 10 ohm on-load voltage...
No information is given about the transformers in this circuit suggested by Motorola in a 1968...