T-Mobile USA My account has been assigned to Receivables Performance Management for payment processing and collection Albuquerque NM

At October, 2007 I subscribed the service from t-mobile.
According T-Mobile return policy I sent back devices in 2 weeks and cancelled the service from T-Mobile by phone.
In spite of this T-Mobile sent me bill for equipment (2 phones) and monthly services.

I called Company and asked why they charged me?.
Representative answered, that I returned phones, but didn't cancel (!) the service. The question is: why I will return phone and still continue the service.
Figure out I really continued the service, but it means I should activate another phone.

It was never done!
The most interesting that one month later I've got new one bill from T-Mobile. The company continued to charge me without any using minutes.
I called again. Useless. Representative insisted that I should pay for service.One year later I still have calls and bills from T-Mobile.
How long it will continue?
I have copy of UPS return label.
Please HELP

Lev
North Port, Florida
U.S.A.