Every Openreach line has a test facility which can be called using the number below. This will read back the line number to you. This can also be used whether this is a live line, or one that is stopped. As long as it has a dial-tone this should work.
Dial: 17070
NOTE: Customers who withhold their number should dial 147017070.
This facility may provide you with options to complete diagnostics. We recommend that you DO NOT use these facilities as they are designed for use by BT Openreach engineers only. For any faults please contact support.