Talk and text roaming

[Armie]

Which roaming toggles do I need turned on for , talk and text only, in USA

[slusagm]

US needs VoLTE to have voice call, so it is generally advised to have Mobile Roaming data enabled on your phone.

and since PM is prepaid and you need to pay addon ahead, if you don't have roaming data (and just have roaming talk/text) ,   there is no chance that data will be used when you are roaming in US, only voice and text will work 

[golfball]

You can enable roaming freely on Public Mobile as it is a prepaid service and you will never get any charges after the fact. You just need to make sure that you either have a plan that includes US roaming or have purchased a US roaming add-on if you don't.