The answer would be for your father to contact his service provider, which appears to be BT Business, raise a complaint and sort it from there. Contacting others simply isn't going to work - they have no obligation to him.
Retrieving a ceased number from the pool is possible but there isn't a lot of time to do it. If it's been reallocated then there's going to have to be some negotiations between CPs to resolve this. Once it's been reallocated the process get a whole lot harder to the verge of being impossible!
You might need to push the complaint as high as you can, but BT Business also have a very good customer forum which could be a good start.
Like I said, nothing is impossible but getting it done is a different matter.
