unsatisfied customer

I have been cell c' s customer for more than 6 years, last year I upgrated to a Z10, the phone had factory faults and I took it to the store more than twice it was not fixed properly, eventually they changed the phone and gave me another Z10 and its the same story the phone is not working.I can't keep on travelling by taxi every time to go to cell c and they keep giving me phones that are not working.why are they not stopping the payment because if you skip a payment they deactivate immediately. What am I paying for?am tired of calling cell c.ga!