Bad customer service MTN

I cancelled a contract in April and did everything that customer service told me to do and down the road 3 months later still get billed for the cancelation contact and my contract got blocked because of it, I investigate it and after numerous phone calls 2and a half hours later get helped and got told it will take 14 working days for the credit and still get billed for the cancelation contact again, why dont people do their work???