Facetime from Overseas

I live in Ohio but travel several times a month to Hong Kong. Facetime worked great while in Ohio to my wife's iPhone. The last time I travelled to HKG, I only put my phone to sleep in airplane mode during the flight, and then woke it up while in HKG to make facetime calls to her over wifi there, and it worked great. I had no mobile provider indicated while in HKG (I guess I need to activate roaming later on).

However, once I turned my phone off and then back on again while in HKG, the Facetime quit working and the facetime option for my wife from "Contacts" was gone. I tried everything, including turning facetime off and then back on again, but all I got was the dreaded "Waiting for activation" note. Once back in the U.S. with a mobile signal and wifi, Facetime worked again. What gives? Is there anyway to turn my phone off overseas and then turn it on again and have facetime work? I've read about the phone needing to send SMS messages yet I've also read it only needs wifi signals (like while on an airplane). I can attest that if it only has wifi signal from STARTUP, while either in Airplane mode or without any mobile coverage at all while overseas, Facetime won't work.

It's like it needs some sort of mobile signal when it powers up to activate Facetime. Any help on getting the phone to stay Facetime-able while overseas on wifi only after a power up? Thanks!