The BT5 is a free standing, self-powered unit, with a full British Telecom approval; S/2455/3/H/4522969. It connects a single amplifier within the PA system to the telephone system by plugging into the extension socket where a conventional telephone would be used (the BT5 telephone jack plugs into the two-wire master socket). The BT5 connects to the amplifier input using screened cable which must be grounded at both ends, with the correct output selectable from line to microphone level, with priority control available where required.

When this extension is dialled from another handset the ringing current is detected and the BT5 will loop the line. An acknowledging tone, and with the MIM33 optional model; a chime, is sent to the caller indicating that an announcement may now be made, which will be conveyed over the PA system. Once the announcement is complete and the caller's handset is replaced, the BT5 will disconnect from the PA system and release the interface extension, thus making it ready to accept another call.