I've been reading this post and decided to jump into dissent: I've switched to Android (because run tests on various phones) and to get a fully immersive experience, switched my main SIM instead of keeping the test one. I hate to rain on anyone's parade but I've come to the conclusion that this IS BY DESIGN, maybe since iOS 7.
I still have my iPhone, and I associated it with the test SIM number, therefore "cancelling" iMessage and all that jazz from my phone number, technically.
Even then, people that had me previously in their contacts as iOS client cannot reply to ANY of my texts. Why? Because the option "send as SMS if iMessage is unavailable" was deactivated for everyone. None of them got the popup to offer to send SMS when it failed, save one guys.
Bottom line: send as SMS seems deactivated by defaut and unfortunately, I have to coach everyone I know on how to do it, because most of them aren't tech savvy enough to troubleshoot this on thwir own. Annoying? UNDERSTATEMENT.
What's ever stranger is that seems this is by design: my girlfriend's sister has always been on Android, and she never had issues sending her SMS even though that option was disabled. I assume it's because her iPhone never "understood" that phone number as being an iOS device. But my number was at one point, thus the iPhone is assuming blindly it will always be associated to iOS...
I've swtiched my main SIM to an Android before, back in the iOS 6 days, and I never had issues like this, and nothing is different on my end.
The last part of my evidence is the first night I switched; I took my SIM out of my iPhone and into the S4, but I didn't put the test SIM in the iPhone right away, thus it was no SIM, but still connected to Wifi. But... every single iMessage sent to me got to my iPhone without a hickup. It's only when I put the new SIM in, thus dissociating my iPhone from my number, that the problems started.
For all I love Apple products, this is pushing the ecosystem buy-in a bit far... I still use iMessage on my iPads and Macs, but this Android phone - aside from being a necessity to test for me right now - is starting to grow on me. It's too bad that Apple seems to force our arms.
iMessage is broken, and needs to be fixed quick. It's not up to the stardard Apple seems to put it at, and they need to re-think how they deliver their services. IMO.