I've also observed via packet capture that the Wifi Calling uses IPSec to connect to telco's Wifi-Calling-Server.
I've tried a few things:
1) While in at home in SG, Wifi Calling still work when I turn on airplane mode while leaving the wifi on.
2) I've tried doing the same overseas but it didn't work, which led me to believe that the Telco's IPSec server is rejecting IP addresses originating from overseas.
3) I then tried to enable VPN on my phone to route all traffic back to my SG home, in the hopes that it'll make it seem that I'm connecting from SG, but the Wifi-Calling-IPSec traffic went directly to the wifi router's network rather than my VPN tunnel.
One last thing I've yet to try, bring my own wifi router overseas and create a site-to-site VPN from overseas to my SG home, set the router to route all traffic to the VPN tunnel back to SG, then use the wifi on that router.
Haiz.. so much effort just to make cheap roaming calls.