Double Debit

I have a Post paid Mobile account with Telkom Mobile.My Direct debit did not go off for the May Installment.i called Telkom mobile to find out what happen on 3 occasions.They were unalbe to tell me why it has not gone off.I spoke to a consultant who advise me my account was in credit and that i will NOT have to pay the May Installment.I was still concerned about this as i did not pay extra on any occasion before.I called again and was told they not sure what has happen and that they cant see any problems on the account.Thereafter i called for the 3rd time and requested to speak to a manager, i was put on hold and thereafter was addressed by the same agent and wanted to know what was the problem again, i was then put on hold again for another 5 mins, and then was told they cannot see a problem and my account was in credit and i will not be charged for May.i was perfectly happy with that, until the beginning of June i received a bill for the amount of R600.00 stating my May and June installement.i have still not received any clarification regarding the bill.