I don't understand what you're asking in your prior post when you say whether you could "pay someone else". Your carrier makes the agreements about with which others your phone could roam (and with who it can't), and your phone tries among the ones it can, but clearly not among the ones it can't. You continue to pay your bill to your home network. And yes, there are often restrictions with how much you can roam.

That being said, TMo and Sprint are making TMo sites advertise an old Sprint network identifier to Sprint phones, so that Sprint phones don't even think of it as roaming [1]. This is in effect for data only right now, not VoLTE.

[1] https://www.reddit.com/r/Sprint/comments/96lai0/clearwire_b2...