It could be the 1571 messaging service.
If someone leaves you a message, BT's computer will periodically call you to say "You have a message. Please call 1571". If you don't get to the phone in time, and call 1471, BT's computer returns your own number as that of the caller.
Remember that the dialling tone is an indicator of whether you have messages waiting. If so, it changes to a two-tone sound (a bit like a fire engine).
There may also be other calls from BT which return your own number when you dial 1471.
Chris