... or any other carrier that supports Wifi calling. I live in Australia. My US Mobile sim "just works" for both inbound and outbound calls and SMS. When I'm somewhere with Wifi coverage, it uses that. When I'm not, it piggy-backs on the data coverage from my other SIM. All for $5/month.

The only annoyance I have with this is that my phone 'only' supports using 2 SIMs at once. So that means when I travel internationally and use an eSIM/local SIM for data, I need to choose between having my Australian SIM OR my US SIM as the 2nd Sim for voice/SMS - I can't have both. But that's a fairly unique problem to me, and most people wouldn't need to have 3 SIMs active at once.

I do agree that if you're using T-Mobile and have a plan from them with good international data options then this is of course going to be a better solution that most other options. But almost certainly at a cost over what you can get elsewhere.