Vodacom won't release my prepaid number for porting

I used to have a contract with Vodacom but have been on prepaid for the last 3 years. I would like to open a contract with MTN and port my number but Vodacom refuses to release my number. The porting agent at MTN says that whenever they try to port my number they are told the number is still on contract and receive the error "****4". I have been back to Vodacom countless times, along with the agent from MTN. Their system says that I am on prepaid and they just shrug their shoulders and says they cannot help me as their system says my contract was fully paid up and I am, indeed on prepaid. They refuse to log a call with their technicians to release my number.

I do not want to change my number for various obvious reasons, but at this point my number is being held hostage by Vodacom and they refuse to fix what is most likely just a glitch in their system.