@Corella1 Welcome to this user forum.
Its possible your line is crossed with another. If you can plug a phone into the socket, and dial 17070, you will get an announcement telling you your phone number.
Check this against your phone number shown on your bill, to make sure its the same.
If its not, then make a note of the number you get, and let BT know, so they can investigate.
That is the only way to prove it. A cheap wired phone cost less that £10.