One the debt is sold to a collection agency, they own said debt, and it is out of T-Mobile's hands.
You can technically pay the debt with us depending on when the account when to collections, but I don't suggest it.
When an account goes to collections, the account is no longer ours. While depending on the age of the account, if you pay us, the money still goes to the agency, there is just the added step that we now have to report it to the agency, and then the agency has to put it in their records. If you are looking to get something off your record ASAP, paying the agency directly is the faster route.