Cancellation of service

In February 2017 I telephoned Telkom to cancel a telephone no longer required. All details were taken and I was told someone would telephone me (this NEVER happened) and I was to complete a form to cancel. I did this and e mailed to service cancellations. Every month I continued to receive an account and for many months I emailed explaining that the service was cancelled. I have now received an sms from mbd credit solutions. I do not consider that I am responsible for this account having attempted to cancel it for 10 months. What shoddy service from Telkom!!! How can they tell me I am still responsible for this account!!! The telephone has not been used since December. Please somebody help!!