[Samsung Galaxy Note 8 Questions and Answers] Kamikaze_Ice:
I know AT&T generally try to tell people with non-AT&T phones that they don't work properly, if at all, on their network.

I chose to play it safe and tried to avoid talking about things like this when calling in.

First get your IEMI from the phone (it's in Settings > About phone).

Second, copy the number down that's written on your sim card (20 characters long). This is the sim card id, aka ICCID.

Third, make sure you get an AT&T CSC on the phone first as the CS rep will have you check for Enhanced LTE services being enabled and you need to have the option to toggle Wifi calling on/off in the same menu. If you can complete the Wifi calling setup (registering a default 911 location) then it will work, which the rep will ask about.

Finally, have your phone off before calling.

*Tip: Sound cheerful and be polite (ma'am, sir, etc). Try to sound as stupid as the last call they had (aka their typical stupid customers).

I called 800 331-0500 from a different phone line and told the stupid automated operator thing I wanted to activate my sim card and played along with whatever questions it asked until it passed me to a real person.

Once with a real person, I told the CS Rep the following:

"I upgraded my phone a few days ago however I can't make or receive calls and texts, but data works. I searched the AT&T's customer support forum (forums.att.com) and was told I needed to call this number and have my sim card provisioned for my new phone. Something to do with an IEMI"

From this point you will probably get asked a few things like...

Where the sim card came from? (previous phone for mine)

What is your new phone? (Say Note 8, don't say any model number or country or where you bought it from.

Where did you purchase this phone? (Directly from Samsung.)

If the rep tries to tell you your phone is not compatible or that the extra features don't work on non-at&t phone or other crap like this. politely tell them that the answer you got in AT&T's "customer support" forum told you to call in about this provisioning thing to fix it and that a number of their customers have already done this and it works. If they didn't do anything to help, this should finally get them to take action.

Then I just do what the rep asks.

You should be able to START the wifi calling setup, and if they did their job you can complete it.

If you can't, let them try again. If they still fail, they will either forward you to another tech or you can just lie and say it does so you can hang up, wait 15 minutes, and call back to get another rep. Just keep repeating this until you can finish wifi calling setup.

When done, wait about 15 minutes, and call back. Tell electronic operator you want to activate again.

When you get to a rep, mention that you called in a while ago and you just want to confirm the 911 address on your record for provisioning.

Now everything should work.

If not, check your plan. I had an old grandfathered 2GB plan that didn't have HD VIDEO, which is needed for enhanced LTE services as far as I know.

I upgraded to a 6GB plan and SAVED $40 a month and gained services... lol.

**Note: Their "Customer support" forum (forums.att.com) is literally customer support. As in other customers helping other customers. I don't think any tier 1 reps know this. And if they do, Customer support forum is really customer-to-customer support, as in peer-support. The forum is official, but any "support" is about as official as anything XDA forums.