T-Mobile is wiping out some debt owed on phones purchased from Sprint
Or, as one Redditor with the user name “xzxnightshade” explains, “Basically a small portion of Sprint customers never transitioned to T-Mobile for whatever reason, and the phones they got owed money to Sprint. They (T-Mobile) decided to get everyone migrated over. T-Mobile would buy out the money you owed to Sprint, and once it appears on your bill they can migrate you over fully to T-Mobile, it’s like unlocking your phone in a way.”
If you did receive one of the notices, don’t waste your time analyzing it. Enjoy T-Mobile’s largesse; for whatever reason the carrier paid off your debt. Congrats!