I too had this issue with Verizon. I was over-charged month after month, and was always told that it would be "taken care of" I never went over my usage or minutes. Reps would tell me one thing one day, another thing another day. After months of being charged double and triple what I agreed to, I cancelled my contract and got charged the early termination fee.
I absolutely refuse to pay the early termination fee, as it was their inability to maintain their part of the contract that forced me to cancel. They sent me to collections, and I'm disputing the debt. I believe that you'd be in your rights to do so as well, but you'd of course need to check first. You can do some research on the internet, there are lots of websites that are dedictated to the horrid customer service that Verizon has, some have instructions on how to deal with this (try verizonpathetic.com, that's where I found the most useful info). Unfortunately, it seems to be a fairly common problem. I will never, EVER use Verizon again.
Good luck to you, when dealing with Verizon...you'll need it