Update from USA
Airtel latches to AT&T in USA. AT&T has discontinued 2G w.e.f. January 2017 - only 3G & 4G available
Airtel IR Pack 2,999 works fine in USA [10 days validity; unlimited incoming calls/SMS; 250min local/India calling; 100 SMS per day]
T-mobile 2G/3G/4G is available. Airtel also latches to T-mobile 2G [when AT&T isn't available], but service is patchy!
Moto G5 Plus [India model] doesn't seem to support WiFi calling. however, it is compatible to AT&T 3G & 4G