Here's our solution to the mobile phone question.
Requirements:
1. during 12+ months of travel, keep your USA number
2. send and receive calls & texts on USA number, wherever you are
3. minimal hassle
4. reliable 3G+ data via cellular, wherever you are
5. four lines
Our Solution:
Plan: T-Mobile One plan, 4 lines for $140, plus local network prepaid SIM cards if we settle in for a month somewhere
free unlimited cellular data worldwide
CON: $0.20/ min for calls (but we don't call much)
CON: locals have to call our USA number (too bad :). A small chance that TMO might decide we are abusing their roaming policy by having all our calls be on roaming networks for a year.
Phones
Helen and I: Moto Z Play International: awesome battery life, dual SIM, $300.
Kids: Posh Micro X - teeny-tiny android phone - too small to play games on, but has camera, google maps, text, etc.
Other reasonable sounding solutions
1. google voice, plus local SIMs.
2. Google Project Fi (google's cellular service, free unlimited worldwide data and voice, but you have to buy $600+ Pixel phone)