I have 3 devices of a
Realistic 43-218 Selectacom 3 Channel FM Wireless IntercomDevices.
They are mains powered and also use the mains to transmit the signals
between the devices.
The instructions are similar to these
https://www.manualslib.com/manual/137506/Radio-Shack-43-491.html?page=4#manual
cheers
pam