No Fi switches between 3 cellular networks and has overseas roaming partners it can do the same with. You can also use wifi calling and since it replaces your google voice number you can use hangouts as well with your number.
Republic is wifi calling first and switches to cellular when wifi is not available. It is tmo or sprint not both. I think new lines are tmo. It is supposed to do it more seamlessly than regular wifi calling eg if you leave the house the call won't drop when out of wifi range. You have to have their app and there are limited devices it will work on. Sim swapping is therefore limited.
I believe Republic is "WiFi first", defaulting to WiFi when available.
"Wi-Fi networks are used as the primary data and connectivity source to the network, while the cellular network is used as a failover if no Wi-Fi networks are in range of the device. The model relies on a proprietary VoIP application for the Android operating system that has the ability to switch quickly between Sprint's CDMA mobile networks, free roaming, and WiFi, depending on which access network is available"
A very limited range of handsets is allowed, presumably because of the need for the hand-off between Wifi and cellular.
Although I haven't investigated it in a while, I believe there is no "T-Mo + Sprint", but rather either/or, depending on the plan.
(Edit: sorry for repeating Isamorph's info, not seen while typing. The update is "what he said".)