Listed at ITC after cancellation of contract

I notified Mtn that I would like to cancel my contract in August last year. Customer Care even rep**** and asked me which cell phone numbers I made use if, which I then provided. I was never informed that there was any amount outstanding. I went to the bank on Saturday to apply for an overdraft facility and was told that I am listed at ITC due to outstanding amounts to Mtn. I have tried to contact the call centre but as soon as I type in the cell phone number I used, I am told that it is an invalid number. I also tried to contact all Mtn stores in the telephone directory but there was no answer. I will appreciate it if anyone from Mtn can contact me urgently.