@Lawrence
So this seems to be a pretty common response, there doesn't seem to be many people who understand that this is just how it works. With Wifi calling enabled through your provider it ABSOLUTELY works, apple to android, android to apple. If both are on airplane SMS works back and forth with zero issues... except in this iMessage case where sometimes it's flaky.
"it does not and cannot use Wi-Fi" is just plain false. Maybe it's a Telus thing in canada, but it DOES and CAN use Wifi. Let me put it this way, if it doesn't work, then how have my wife and I been exchanging SMS messages between operating systems for well over a year on Airplane mode, have we been imagining it?
I even talked to Telus (my provider) and they were unaware that SMS worked through the wifi calling standard as well. I'm not trying to /r/iamverysmart you, I'm just telling you it works just fine on a technical level, so you might need to adjust what you think should be happening.
But I guess fundamentally there's no resolution here because since nobody thinks it should work period, nobody can assist in resolving, even on the apple level.
Steve