Why my account is not getting settled and Tmobile wants me to call the debt collector agency?

Hola, 

T-Mobile has the best customer service ONCE YOU ARE THE CUSTOMER, but if you leave them,  you gets the worst customer service ever. 

I have decided to switch the carriers last year and gotten a shipment box to return the Signal booster device R3, which I did on Nov 7, I have called them few weeks after still getting the bill for $306 (Signal booster charges) they told me It has been received but the system is not updated yet so need to wait. Since then, I have called more than 15 times (on average 40 mins a call) in past 5+ months. But still getting bills and the worst part is every time I called, I m connected to overseas call center and their default answer is "Oh offline team needs to update the system" and they cant help further. Now it has went to collections, which I told the T-Mobile team several times in the past don't send to collection and they said since you call us T-Mobile is not going to send to collection. So called T-Mobile again today and was told we cant do anything since its already sent to collection just talk to the collection agency and tell them T-Mobile has received the device and please settled my account. when called collection agency they said they don't take customer's words, T-Mobile needs to let them know.  NOW I M STUCK IN INFINTE LOOP!!! Please assist.