MTN Breech of contract

on 17 Feb, sent my Blackberry in to MTN for repairs. Was told would take 7-10 working days! Finally received a refurbished device on 9 March - well over the 10 working days. This device wouldn't even switch on & kept rebooting so I took it back on 10 March. Its now 20 March and I am still phoneless. Spent endless hours at MTN - nobody can help me. Every time I call they always transfer me to repair centre. At repair centre they no record of the second device being booked in and dont help me at all! It was within 6 months of taking out the contract that phone went in for repairs. I tried to demand a replacement device that is not Blackberry as those phones give endless troubles. According to my contract I can cancel or ask for a refund or replacement. However, I am getting nowhere. It over a month since this nightmare began! Repair centre say no records, then its we doing stocktaking this week, etc. Endless excuses. But repair centre number is call centre, how can a call centre be doing stock taking while the devices get sent elsewhere? I'm disgusted with MTN and just want a replacement device that is agreed upon between both parties at same value as current device!