B.T. is the line provider
Older phones may no longer work on your BT Retail phone line, especially if they do not support tone dialling. Most phone now only have two wires, and are tone dialling only.
If yours has more wires, then it may be an old dial phone which has a mechanical bell and a rotary dial, and has a separate "bell" wire
It would have to be for UK usage, as the DTMF tones that are transmitted would have to match the BT exchanges.