As long as SMS relay has been available, I've been able to make it work with all my devices, including my non activated iPhones. Meaning I was able to send and receive green bubbles using any/all of my non activated iPhones with my current carrier activated iPhone being the hub as long as I was connected to WiFi, currently an iPhone X. As of late (maybe since 11.2) I've noticed that I can't send from my non activated iPhone 6s Plus, only receive. If I send the green bar hangs eventually I get the message "text message unable to send". My 6 Plus on 10.2 has no problem sending or receiving. My X and 6s Plus are on the current developer beta of 11.3. I've played with the settings, turning off/on FaceTime/iMessage and the relay on all my devices. Still no luck, not sure what I'm missing or if its a software thing with iOS itself?