Holding mobile number hostage

I am so disappointed in the service that I have received from Telkom.

I was on a Telkom mobile contract for 24-months, I canceled my contract and submitted a request to port my number to prepaid.

After numerous calls to the call-center, no one was able to assist me. It's been 2 weeks and I received an email stating that my mobile number will not be released due to it being a contact number on portable ADSL device/account, that is not mine.

I don't see how they can hold my mobile number hostage for an account that I am not liable for?!?