Yes in theory. But you can't roam the entire time or everywhere another provider has signal; in fact I've never managed to use it because I've only tried it in locations where my provider has awful coverage, and my phone's been unable to register. It's clearly only tailored to allowing you to get some semblance of service when you go out of the official range on occasion. It has various limits and T-Mobile terminates your contract if you roam more than half the time. So it's not like you can live somewhere out of their range and just change whom you pay your bills to and switch to roaming most of the time.
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.
I'm saying their agreements are not equivalent to being on the other carrier because they might not help you in the particular time/place/type/amount of roaming you're interested in. Hence claiming it's all LTE because of the existence of roaming and non-existence of SIM/band compatibility issues with recent phones still paints the wrong picture that carrier doesn't affect where you get reception, whereas it very much does.
