MTN Service disaster

I have a sim card which is not making calls, MTN failed to help me into working properly, I am receiving airtime on the sim card which I cannot using but I am being charged for. I then asked them to cancel the contract and now for 2 weeks they are making me run around in circles. They told me to call the call center, call center gave me number which is not working, I went to the store, the store tell me to call call center, I called call center again which told me they helped me and they will call me in 3 days time, after a week they haven't called me when I follow up they tell me I must send an email. This is ridiculous and unacceptable, they receive their money on time every month.