breach of contract

I was sold a contract phone at R190 for 24months by Homemark on behalf of Vodacom. the first month i was billed incorrectly, i called and they said there was an error in the system and they rectified it. two months after the same thing happened and it and they corrected it. Last year November I received communication from Vodacom stating that I was incorrectly billed for the past 6 month and I would have to pay R270 instead of R190 sated when I took out the contract. I queried the communication for a month with little to no help. today I got a communication from Vodacom stating that my contract states that I have to pay R190 for 24 month but there was a glitch in their system at the time the phone was sold to me and that I would have to pay the R270 as there is nothing they can do. so now i don't know what to do because they are in breach of contract and are forcing ,e to pay the money although the fault was not on my side