False claims from Vodacom call centre agent

I talked to a Vodacom customer services agent today expressing my wish to port my current Vodacom phone number to another mobile services provider. My current phone number is long past the period of a locked-in 24 month contract with Vodacom and is currently running on a month to month contract. The Vodacom agent said that porting my number will not be possible and that no process exists to port my number now or in the future. That's such a bunch of BS. The number I refer to is an old 083 number from MTN which I ported some years ago from MTN to Vodacom. Now the Vodacom representative tells me that there is no possible way to take that number back to MTN. I feel the Vodacom rep is talking *******! I would like for Vodacom to respond with the reasons that Vodacom claims it will not be possible, now or in the future, for me to end my month-to-month contract with them and sign up a new contract with MTN together with porting my current phone number.