Wrongful billing, horrible customer service

I have been battling with MTN for three years now. They still send me SMS messages that I owe over R3 000, but I cancelled my contract in writing and sent them the proof of who I spoke to with proof of payments. It is still dragging on. When I phone customer service, I hold on for more than an hour, just to be transferred from one consultant to the next who can't assist me. Your customer service is non existent!