My recommendation would be to call a corporate TMobile store in your area (Not the store you are making your payments at if it is, or it may be a satellite store, you didn't mention I don't think) and find out who their regional manager is, and a way to reach them. It sounds like its an issue with a certain employee or employees at that store.
I have experienced problems like this before with Sprint when i was a customer with them. They kept trying to charge for long distance calls (not roaming), saying it wasn't included in my plan. I finally got ahold of the regional manager and got the issue resolved-for that moment. 3 months later they tried to jerk me around again and I dropped their service. Fortunately I was at the end of the contract by that point so I didn't have to pay a cancellation fee. But you are right, threatening cancellation is always a way to wake them up. Thats what happened when my local store refused to sell me the 8700.
I have experienced problems like this before with Sprint when i was a customer with them. They kept trying to charge for long distance calls (not roaming), saying it wasn't included in my plan. I finally got ahold of the regional manager and got the issue resolved-for that moment. 3 months later they tried to jerk me around again and I dropped their service. Fortunately I was at the end of the contract by that point so I didn't have to pay a cancellation fee. But you are right, threatening cancellation is always a way to wake them up. Thats what happened when my local store refused to sell me the 8700.