This power supply protection circuit using a relay was found in a 1967 documentation. The components are from the time, but current equivalents can be used depending on the power source current.
This simple receiver for listening on the phone was obtained in a 1960 documentation. The circuit is...
This circuit is from an American publication from the 60's. The power is on the order of 600 mW....
This circuit was obtained in a 1980 English magazine, but it can be easily assembled today, since...