MTN in breach of their own contract

I signed a contract (a special that was running at the time), for R259 a month. I noticed that they were charging me R200 a month more than the contract and asked to be refunded. They have overcharged me with R1400 and then only put a R500 odd credit on my account. I pointed out that this was wrong and that my bill has not been adjusted. The reason given was that my work telephone number is wrong (which I corrected when I took out the contract), and per email and still they refuse to refund me and have actually sent me a SMS that I need to pay another R500 on 25 February despite the fact that they owe me and that I choose that my account be debited on the 7th of each month. I am on the brink of getting a lawyer. This is a last attempt at wrenching some service out of this company before proceeding to legal relief.