Contract cancellation

My wife's contract phone was damaged and it was not insured. We decided to pay it off 1 year before the contract expire. We requested a quotation and we were told to pay the balance within 11 days. Payment was done and all the required documents were sent the same day we received the quotation. After a month we received a call from Vodacom that we skipped a payment. We told them that the contract has been cancelled and they requested the proof of payment and we sent it again. We checked with them 3 days later and they confirmed that the contract has been terminated and they sent us the confirmation statement with zero balance. Yesterday my wife was called by a collection company saying her Vodacom account has been handed over to the lawyers. When we call Vodacom to find out why the account was handed over we get transferred pillar to post and no one is able to tell us why it was handed over