Accused of non payment of account

I received a call from the collections department of MTN and was asked why I was in arrears with my 3G account. I have had a running battle with MTN for months over my account. I signed a debit order agreement with them years ago and there is always sufficient funds to cover my account. When I asked why they do not just debit me, they could not answer. On checking it was found that my account had been debited and on further investigation by me I found that I am in fact in CREDIT to the amount of R250. I sent a query to MTN asking them to investigate and report back to me. I received a computer generated answer thanking me for my message and saying they would get back to me within 8 working hours. Well MTN it is now three days later and nothing. MTN, check your facts before calling me and accusing me of not paying my account when I am in fact in credit<br>