I created an account here just to comment on this thread, as this issue nearly killed me (not literally)... and may I can help some people out? IDK...
For context, I'm in Canada on Virgin Mobile.
I received my new 6s on Friday, and my husband and I had planned to switch service providers as soon as we got the new phones because we wanted a bigger data package. As soon as the SIM was installed I started having issues with iMessage. After searching the internet for hours, I discovered that my new provider (Wind Mobile) doesn't support iMessage because they're not an authorized dealer (also, their service sucks). So, back to Virgin I went.
Then, everything was great but iMessage still wouldn't authorize.
When I restored my phone to port the settings from my old 5s to my new 6s, I did it using iTunes. I first backed up the old phone, then restored the new phone and everything seemed to go well with that. When my husband restored he did it using iCloud. He had been able to use iMessage with no issue all along, while I was still having issues.
I factory restored, recovered, etc twice over the course of this passed weekend. I spent hours reading forms to figure this out, and my husband still had no issues. So, with the only difference between my phone and his was the way it was restored from a backup, I decided to wipe the phone and start from fresh using no backup at all (Ugh..... )
I did that last night. Tested iMessage. It would authorize, and I would try to send a message, and then it would suddenly no longer be authorized.
I gave up, plugged it in, and walked away for the night. In the morning I noticed it was authorized again. So I asked my husband to send me a message. The first one came through as a text, and then the next as imessage. And now I have no more issues.
I hope this helps... I literally tried everything all weekend to get this to work. So terrible.