Hi I have a problem with Mtn contract sim

Hi I have a problem with Mtn contract sim. I have been paying every month and from month September I have been told I haven't been making payment for the past 3months so from my side I made means to visit mtn shop as soon as i got the opportunity too. I was told to get notification from the bank that shows I've been making payment than I did that and I was told everything is cleared now im still receiving calls from them saying I owe payment for about 4months now. I'm really feed up I can't be always visiting the mtn branch offices because it inconveniences my work schedule. Im also tired of explaining the same thing from different agents. Now I'm told I have been handed over to the debt collectors. I am unhappy with Mtn. I wouldnt recommend them with bad services.