The BT range consists of three products:

BT5 Connect

BT8 Interface Phone pager

BT10 Universal interface module

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.

The BT8 is a standalone interface between a PA system and a telephone system. It is not designed for direct connection to the telephone network but under approval NS/G/23/J/100003 it may be connected to a port of a telephone switch (PABX). The output is at microphone or line level via a 25-way D connector that is compatible with the Baldwin Boxall BDM range of desk microphones and plugs into a BMS25 socket box (supplied with the BT8). The unit has a recorded message facility (up to 55 seconds message length) and is designed to work with both MF signalling and non-MF telephone systems, although with reduced functionality with the later.

The BT8 is wired into any two-wire analogue extension port via a designated master extension socket. It can also be connected to a paging port, as available on many modern telephone systems. It is not designed to operate with key phone extension ports.

The BT10 is a universal PA to telephone system interface whose input connects to either a 100V line or the line level output form a telephone switch. The output at either microphone or line level can be fed to the PA system's amplifier. When a sufficiently strong signal is received at the input, the VOX circuit will operate passing the signal to the amplifier input. The VOX circuit is designed to ensure all unwanted noise is eliminated fro a clear and intelligible signal via the PA system. The VOX sensitivity can be adjusted on-site, as can the systems gain which is factory preset to X2 (6dB). The unit can be powered from the mains or 24V DC supply, and where both are connected in the event of a mains failure the unit switches to the 24V DC supply automatically.