Both Wifi-Calling and VoLTE are not supported by AT&T for most unlocked non-at&t branded phones.
They tend to support it only on the major unlocked flagships that AT&T doesn't carry like the Pixel 3. Of course, iPhone is an exception and is fully supported since Apple tends to get it's way unlike other manufacturers.
There is really no way to get them to enable it either. Wifi-calling and VoLTE are controlled via the IMEI on AT&T. If the IMEI is not on the approved list, then it won't work. AT&T will not add the IMEI either. Flashing the AT&T firmware does not circumvent this either.
I had the same problem with my Note 9 that was unlocked and I called AT&T and the tech guy did something with my IMEI and everything worked WIFI Calling, Video Chat and VOLTE. So now I purchased the Note 10+ and it's been a battle for the last three days with them. I flashed the AT&T firmware to get the wifi calling icon on the phone and after being persistent I have wifi calling , but not the video calling. I plan on calling back and trying to get the video calling also.
What number did you call? I'd like to have this function on my phone as my apartment doesn't have great coverage. My AT&T locked S8+ had this function and it's quite annoying to be without it on a newer phone.. I only bought it unlocked for the blue color T_T
Thank you for the information!! I really appreciate it.
I was about to explore flashing the att firmware, but now I will explore other options.
I found a ATT microcell device on ebay, will see if that works.