i have never had luck getting a Sprint phone to work on AT&T or Tmobile, and Sprint assures me it will and after hours of trying to figure it out i gave up.
Maggie are you referring to an S8? I went from Sprint to T-Mobile and all it took was inserting the SIM card T-Mobile gave me then a few minutes later I got a notice saying a software update was available so I installed it and it was T-Mobile firmware. Every function including voicemail works perfectly it's going on 2 months and it was much easier then I expected. You do have an unlocked phone right?