Apple SOLVED this for me after 3 weeks of attempts. She said my number was still "blocked" from being ported in and had to verify who I was by texting me a code and then unblocked it. iMessage instantly started working without turning off my phone or anything.
The full story:
I got the number started on Google Voice in order to have a local area code for my business. Once I was ready to start paying for the additional phone line, I ported it into my provider - Pure Talk who uses AT&T towers. Initially, phone calls worked but no iMessage. It would attempt to send in blue, then fail, then switch to green and send just fine. I thought iMessage was the only issue so I wasn't especially concerned to get it solved right away. Then I started getting phone calls from friends and potential clients asking if I had received any of their texts as I wasn't responding. I then learned that I was not even receiving texts from some people, but they were also unaware that their texts were not getting to me. With this being my work number and having new potential clients trying to contact me and receiving no response I was becoming another contractor who does not respond. I called Pure Talk's tech support every few days for at least a few weeks, tried the whole list of resetting and turning on and off that they always make you go through. I actually ended up losing a bunch of photos from the who process which is still pretty aggravating. At one point I couldn't even send or receive texts from my wife for a couple days. I had been on with Apple support while screen sharing and the first person I got could not figure it out either. Finally I got the right lady who knew exactly what they issue was and fixed it in less than 5 minutes. I'm not on social media and not someone who's always got my phone in my face, but this really shut me down for a few weeks. When you rely on a phone to do business when you're self-employed, no communication eventually means no income. Think twice about porting a number in that was originated from Google Voice or anything else that's a land line I guess.