Billing headache

I took a month-to-month router and cancelled within a month due to poor connection in the area. After numerous calls and visits to the Cresta Branch, my statement comes with R2510, for a R399 contract which was cancelled and on time. No one could explain what the billing of R2510 was for, eventually I was informed it is for a router. The router was taken back to the branch upon cancellation of this contract. I called again and I was informed that the billing was corrected. To date, I still receive calls and SMSes that I owe R2510. Can someone please rectify this as I have no existing contract with MTN according to me. The final attempt to get the issue resolved, I was in formed that the contract was cancelled, however, it is the outstanding balance that must be credited (retention centre), they transferred me to billing and collection, until collection could not transfer me back anymore.