When we moved house three-and-a-bit years ago the person who was buying our old house was able to disconnect our BT line without our permission, perhaps a fortnight before the move, simply by ringing a BT call centre. Quite apart from inconveniencing us it made things difficult with the bank, the solicitors, the removal company and the children's school, all of whom had our landline number as first point of contact. Three or four days later our ISP also cut us off because the line was no longer "live".
Suffice to say we took the opportunity to move away from BT for our new phoneline, but despite all our protestations and communications with BT and (eventually) the regulator, the consensus was "these things happen, sorry, here's a month's rental back". This incident strikes me as very similar. With just a phone number and a postcode a third party was able to take all sorts of action against a phoneline that isn't theirs.
Have to say we've had great service from our new phone supplier who seem to have a callcentre somewhere on Mersyside with real people answering the phone who actually know what they're talking about and we now use them for our ISP too. For example, "fixed IP sir? No problem" rather than "what's an IP address? Oh, I don't know about that, I'll have to pass you on to someone else".