Have you tried Google Voice/Hangouts (available in most "places" abroad) since that's also WiFi calling, free calls home to the US - as long as Google - other calls for as little as 2 cents or 4 cents per minute (i.e. Taiwan to South Korea) - works great on our old Nexus 5 (sold/traded on Swappa) - replaced with Nexus 6 and Nexus 5X running MM 6.0.1 (both with integrated support for WiFi calling on T-Mo.) Next move for us is to switch over to Project Fi since we already got the hardware.

Also, works on unlocked iPhone 5S, running iOS 9.0.2 with T-Mobile WiFi calling and Google Hangouts calling (although the Voice/Hangouts Apps have a different touch/look/feel to it) - it's a prepaid backup line for voice/SMS only - so it's all over WiFi (essentially, like being in Airplane mode since it has no data connectivity ...)