As of November 24, 2020 this is still an issue. I had it, too. What it appears to be is a MASSIVE communication oversight.
The short answer is: Call your cellular provider. They can turn it on (at least AT&T did for me, and it looks like other providers as well).
The long version is: I have AT&T and I was upgrading my phone from a 7plus to a 12 pro. I paid the king's ransom for the phone, waited three weeks to receive it. In under 25 minutes i was able to flawlessly transfer all data, apps, contacts, etc. just by scanning the image from one phone to the other. EXCEPT: when it came to my iMessages and FaceTime registering my phone number, it just got stuck there, spinning endlessly, saying "verifying" or "there was an error, you're signed out of iMessages, go to settings," which I did, which routed me back to the spinning wheel of iPurgatory.
I googled it and went through EVERY conceivable troubleshooting method multiple times. Wasted over an hour of my life. Finally I called apple support, which took me through the exact same steps all over again, and then the support person said I had to just let my phone sit for TWENTY FOUR HOURS(!!!!) to see ***IF*** it would verify!!! WHAT?? This $1,100+ space phone can transfer all data from old phone to new in under 25 minutes, but it won't recognize my phone number for TEXT, the most critical communication mode of everyone's life?!?!? I was blind mad.
Thank god I found this message thread because I called AT&T and within the space of minutes the representative told me to turn off my new device and he simply "moved my number over" to my new 12. Once I turned he new phone back on, everything worked fine.
There are some big questions and failures of communication here:
- WHY is this not clearly stated ANYWHERE in the box or instructions with the new phone I received? Unlike some who've posted, I did not receive any additional instructions in my box. Nada.
- HOW IN THE WORLD do the Apple "geniuses" not know this needs to happen to activate your number? Asking someone to wait 24 hours for ANYTHING tech-related in this day and age is akin to selling them a dial up modem to surf the world wide web information superhighway.
- WHY is Apple, the richest and most valuable company on the planet, cheaping out and not including the USB-C power adapter with the phone? Come on, Apple, really? After we exist on a diet of Top Ramen and tap water for six months to pay for the phone, you're going to send us to the brink of starvation so we can charge the **** thing? Don't tell me anything about a "carbon footprint." I'm a shareholder, for chrissakes, and I say give us the power adapter with the phone. The thinly veiled eco-excuse is pretty disappointing.