Contract impossible to cancel

I have been trying to convert my contract, which has expired, to a prepaid. I have to get hold of retentions and MTN will not help me unless I call on the "contract phone". This is not possible because it is my son's phone who does not reside with me. They will not allow him to call because I own the contract. Basically, I have no choice but to carry on with the contract?? The branches will not help me.