Pathetic Service

I have been with MTN for close to 4years now with 4 contracts with them, On the 23rd May I made an EFT payment towards my monthly billing then MTN decided to debit me again on the 26th May, then I called MTN to enquire about the refund since i double paid(mind you I paid about 3K) the consultant told me that the additional debit and I get told that they allocated it to my next month instalment therefore I don't have a refund, How pathetic is that, I have been transferred to 4 different department which was Billing department, Collection department, Back office department just for them to tell me there's nothing they can do I must contact my bank and reverse the extra payment I made on my side knowing very well that it wont be reversed. At this moment I am not happy with MTN in a way that I want to pay it off and cancel my contracts with them.