Earlier quoted context omitted.
Or just use Google's Project Fi with no roaming charges (if you are from US).
From what I hear, Fi is by no means perfect, it's only available for a few phone models, and I'm not from the USA anyway.
It is only officially supported on Nexus and Pixel phones, which are great devices. You can get around the limitation by doing some Googling as well if you insist on using a different device.