FNB is refusing to port my number out to another service provider

I have a prepaid sim contract with FNB, and I recently took a post-paid cellphone contract with Telkom and requested Telkom to port my number from FNB. Telkom has been trying to port my number from FNB for more than 5 months now and FNB always declines the porting request from Telkom without any reason. FNB ported my number from Vodacom about 6 years ago without any hustle, but they are now refusing to port out my number to Telkom. They are touting themselves as the "bank of the changeables" in recent "misleading" adverti*****ts. I am going to take every opportunity to expose them in all social media platforms for what they are "the bank of lock-in" as "changeables" to them only mean "change to". I would have already cancelled my sim with FNB if I did not need to use my number, which I have had for about 20 years now.