Billed without service rendered

My name is Lucas and I recently (26th June 2015) put in a request to cancel my MTN contract after 24 months (***). The reason i put in a request is because MTN debited a 25th payment from my account when I only agreed to 24 months contract. I was told that I needed to give them a month's notice in cancelling my contract which I wasnt aware of. On the 14th July 2015 I recieved an SMS from MTN saying that I was credited with R210 airtime and 25 SMS' but when I checked my account I still had the airtime and sms' which I had prior to their sms. I kept checking my balance the next 3 days to see if any extra airtime and sms would reflect. It didnt reflect so i called their 808 line and was told by Isaac that the issue will be resolved in 24 hours. Today is the 22nd of July and MTN has not got back to me. Isaac mentioned that money is debited a month later but in my case, I once changed debit order date and was debited twice the usual fee because of that which means when that happened I started being debited on the same month that I get my airtime and sms.I am worried that MTN will debit money from my account on the 25th even though I didnt receive any airtime or sms from them