Being over chanrged for 6 months and they know

I upgraded my contract in December 2014 and am still being charged the higher out of contract rate. I have emailed customer service many times and each time they come back and say it will be rectified and they will contact me as soon as it is. It is now 6 months later and I am still waiting for it to be rectified. They are quite happy to continue to take the higher amount from me and you cannot get through to anyone on the phone. Disgusting service and client relations! Perhaps it is time I change service providers on my personal and business accounts!