When a user has a broadband only line, any outgoing phone calls get redirected to the test message which does not mention the phone number.
While I accept that a broadband only line shouldn't really have a phone number, it is still used to identify the line, both to the use and BT.
Without this information, it makes it difficult to check if it's the expected line. While it reports "line 1", that is of no use to a BT engineer proving the line.
This has caused an issue on a recent installation as the engineer testing in the cabinet couldn't confirm he had the right pair, resulting in over a week's delay to get an engineer to visit, just to say "yes, that's your line"
If there is some other ID that the BT engineer would know, this could also be used, but is then only of use to the BT engineer.