Extremely poor after sales service

If i could have rated MTN ( - 10) i would have. 2 months ago, my phone gave problems and died. Phone still under warranty. It took 2 weeks just to get an invoice from customer service as this was an online purchase. The following monday i booked it in at MTN Ballito Junction, and today i went there and my phone came back unrepaired,because the invoice didnt accompany the phone when MTN Ballito sent it in for repairs. I want to know how i can cancel my contract with mtn, and stop paying these ******. Or i want a new phone. Period