Im currently having difficulty with a company who we have our 0800 number with, they are trying to play silly buggers and not release our 0800 number so we can port it into our own phone system because they 'own the phone number'

Now, I always thought there was some sort of legal requirement for them to release it should we request them to? 

I've tried googling so I can send him a link saying 'well you kinda have to' but im not a voice guy so my searches aren't yielding much.. is someone able to help? 

Or.... am I completely wrong here?