Billing continued after service disconnected

I had an internet contract with MTN. I had payed diligently for nearly two years, close to the expiration date of the contract until my banking details changed. I went to the mtn branch on three separate occasions to notify them that my details had changed so that my internet connection could be restored. On each occassion they failed to so so. I was given the excuse that they could not reach their debtors department. I was told to come back on different days with the same consequence. If I tried calling the contact centre it also was to no avail as no one answers the phone. My bank also tried and failed in assisting me to update my details. In the mean time I was being billed and now mtn has kindly handed me over to their debt collectors. I feel this is unacceptable as I had tried several times to update my banking details with them. The debt collectors aligned with mtn have now handed this information over to the CPB Credit Bureau, according to a notification received today, even though the email I received stipulates that I have until the 23rd of February. So I'm being forced into paying months worth of internet service I have not had.