Phone number is linked to an old account

I'm trying to help my father-in-law with his Apple ID account. His old iPhone 4 somehow got locked, but he has no idea what his old Apple ID details are. I have got him a new SIM card and put it into an old iPhone 5 I had to get him going and set up a new apple ID for him, so far so good. He doesn't use email and hardly uses the Internet.

However his phone number is linked to the old account and we have no access to it (we are not even sure the Apple ID we think we know is correct) .

Is there any way to unlink his phone number from the old account and add to the new one ?