I have a S6 and Sprint unlocked it for me. It works fine with a T-Mobile sim. The only thing is for the service to kick in I have to make a call first. But I don't get data while on the phone, Hd calls, volte and if I am on a call and once it ends the get back to lte I have to put the phone on airplane mode or else I'll be stuck on hspa+. It works though which is most important. Oh and the sprint lte and 3G logos stay.
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Those are my 4 speed tests that was done at 4 in the morning that are in the 70s