I would try contacting this email address for help:
ExecutiveResponse@T-Mobile.com
Consumerist article about the above email:
Email Addresses: Email Tmobile Executive Customer Service
This is the executive response support line, basically they are premier support. This started with Sprint (we know how lovely Sprint support is supposed to be, though personally I LOVE Sprint) and the concept spread. You work with regular support. If they give you the run around, contact the above.
If that doesn't work, I think it may be a time for an EECB (Executive Email Carpet Bomb). This is your last resort, and assuming everything you told us is TRUE, then keep trying to only pay what you owe.
How to do an EECB:
Complaint Letters: How To Launch An Executive Email Carpet Bomb
T-Mobile CEO Email: rdotson [at] t-mobile.com/robert.dotson@T-mobile.com (possibly go to same place)
The rest will be up to you to find.