QUESTION about a new iPHONE

My wife currently is a Straight Talk user and has an iPHONE 6 activated. My daughter just gave her her old iPHONE 8 Plus that is unlocked from Sprint. I need to get this setup so she can use the cell service. What do I need to do? She seems to think all I need to do is put her SIMS card in the iPHONE 8 PLUS and it will work. I think NOT
Thanks

Thanks