You and everyone else who purchases an unlocked or even a networked locked device outright from the manufacture needs to sue any carrier that prevents wifi calling. The carriers purchase the rights to use a specific microwave frequency from the FCC. The carriers did not manufacture the phones we purchased nor did they pay the government for the exclusive rights to use any wifi frequency. Making a wifi phone call uses hardware make by a 3rd party manufacturer and purchased by the end user and uses a frequency that has nothing to do with a carrier.
I purchased my unlocked s9 directly from Samsung. I am using cricket network. I did not see a place to enable wifi calling. I called cricket and they said since it was not a cricket branded phone they would have to send out an update for my phone. They said I should get the update within 24 to 48 hours. Until then I'm in a waiting period. Hopefully everything goes ok after I receive the cricket update.
The people at AT&T just confirmed factory unlocked Galaxy phones don't support WiFi calling. There's not even an option to turn it on. Never had any use for it, but my new apartment has almost no cell service. I've set up a Google Voice number (which uses wifi calling) and forwarded my calls to the new number. It should work. Not convenient, but way better than all the bloatware the carriers jam into your phones.
To confirm, I bought an unlocked s9+ and used it with Verizon. There was no wifi calling or even settings. Switched to tmobile with the same exact phone, popped in SIM and I now have fully functioning wifi calling.
ATT blames Samsung. They said I should return phone to Best Buy and Purchase through them. Gave me the number to contact Samsung to try and update the phone. Samsung said it was not possible. Only the carrier can do that. The Samsung rep at Best Buy assured me that there were no major differences between unlocked att phone. Here is the chat with Samsung. "Sprint and TMbile are the only carriers that allow wifi enabled calling on unlocked phones." I am done with Samsung and ATT. Maybe try google pixel? not going to apple and dont want Samsung anymore.
I bought a AT&T prepaid phone. I bought the phone with the understanding that the WIFI calling app was installed and available. This is not the case. AT&T help number says WIFI calling doesn't come on prepaid phones, but probably will be available later in 2019 for a price. AT&T doesn't own my phone and should not be tampering with apps that supposedly comes with the phone.
I'm on an S9+ unlocked (non carrier specific G965U1) on Ting (which uses T-mobile for GSM I believe) and wifi calling does work. Though when it's in wifi calling mode, I cannot receive or send pictures. So I leave it in "use wifi calling only when you have no signal" or what not. When testing such things, be sure to check that it is enabled on your phone service account.