Fnb sim hostage

I have been with Fnb cconnect for a few weeks and definitely not happy with their service and i tried to port back to mtn and these guys are reluctant to approve the porting process.I was told to call their customer service on 135 to deregister my sim to make the port process smoother but nothing came of it,why is it so difficult for Fnb connect to let unhappy customers go? If anyone still thinks of using their service my advise to them is "DO NOT"