Cancel Service

Has anyone had success canceling service before your device contract is up? Tmobile lied about the service plan they put me in and keeps incorrectly over billing me. They acknowledged the mistake, but they make me call every 30 days to dispute the charge. I stated I wanted to cancel my new plan inside the 30 trial period but was assured the mistake would be resolved if I stated. I feel like it has turned into theft from them. Has anyone had luck with reporting them to the FCC or The Better Business Bureau?