Deleted phone number from network

I had a number with MTN, we had a fall out due to poor service and as a result terminated the contract. However MTN kept my number and didn't not release it as they claimed I owed the stil. I tried porting to other networks and the number could still not released. I recently realized the the number does not belong to anyone nor any network. And when I ask MTN about it since I would want to use my number again. They claim they can't do nothing. As it stands I can't port nor do a swim swap with them as the number is hanging and probably waiting to recycled. This is really messed up as want my number back and MTN is refusing to help