Handed over for debt collection after contract was paid up

MTN South Africa handed over my contract for debt collection after it was paid up. I was never informed that my contract was paid up until I received the call from the debt collectors to say that I owe over R1000 for I don't know what. I want to close this contract but I'm unable to do so because none of MTN's numbers work; not the one for head office ( ********** ) nor the alternative number the automated voice gives you when you call head office ( ********** 000). I approached an MTN branch and was advised that contract cancellations can only be made at head office. Seriously! In this day and age where offices are virtual! The official at the branch gave me these numbers but given that none of them are working, it means that I must take time off from work to close this contract! Customer service is non-existent at MTN!!!