re: Sprint or T-Mobile?

I have Verizon currently. I'm having some serious issues with them regarding customer service trying to get an issue with my bill fixed. I was charged for $450 worth overages that I did not use. I've had to speak with three different supervisors to try and resolve the matter and have gotten nowhere. I've had all of them tell me they will call me back, but none of them ever did, so I've been sitting around trying to figure out with the hell to do.