Mods: I think it's time for a dedicated Sprint Salaxy 4S forum. Thanks.
Looking at the FCC docs for what appears to be the S7, it seems like Samsung will finally make just a single hardware variant for the US, FCC ID A3LSMG935US, with the different carrier models' features and bands being enabled/disabled solely in so...
This bit of news may just confirm the premise of the OP, but is also bad for those hoping to use one US device on multiple carriers. In a nutshell, the device gets locked to the carrier when the first SIM card is inserted and the device set up:
What you are describing isn't Samsung's fault. Basically, it was your choice to purchase this specific model (815C). You could have purchased the T-Mobile specific model (817T) which has all of T-Mobile's LTE bands or the an unlocked AT&T model (8...
Sometimes it's a good sign - means folks don't have many problems with it and/or it has a decent stock experience requiring less mods.