I use Google Fi; it works great almost everywhere with 4G/LTE service. In the few places I've lacked data, calling and texting has been available (free texting everywhere; phone rates are variable but generally $.20/min). Wifi calling isn't advertised but you can put your phone in airplane mode and turn on wifi for the same effect. Data is $10/GB but you're refunded for any you don't use and there are no overage charges. I use about 2GB of data per month and infrequently make calls overseas without wifi; my bill runs me $45-50.
The downsides: Google uses Sprint, T Mobile, and US Cellular towers for its coverage in the US. If you can't get coverage from one of those carriers at home you'll find yourself without coverage from Google as well. I use mine as my primary phone but have to put it on wifi when I'm at the house to get decent coverage. Anchorage is also a data dead zone on Google so, as an Atlas pilot, that can be problematic (though there's plenty of wifi available downtown). Overseas, the only place I've found myself without service is Luanda, Angola though I suspect much of the rest of Africa is probably the same. Works great in just about all the other cities Atlas goes to regularly.