Pal PhoneBoy, an uber, uber geek has penned a very descriptive piece on the new AT&T Wi-Fi calling functionality.
Yesterday, I went to the AT&T Store here in Miami and updated my SIM to a xxxx27 version from an xxxx25 edition that dates back to my iPhone 5 era as I've been on NANO SIM's since the day they came out and nothing from AT&T ever came along suggesting an "upgraded SIM." (I guess this is one reason why Apple is pursuing the idea of a SoftSim concept.)
My experience was fairly similar to my first day on T-Mobile Wi-Fi. Once you get onto a Wi-Fi network the indicator changes from AT&T to AT&T Wi-Fi and where you once didn't have great coverage, you now do.
All my calls went through perfectly, but there's been no one on the other end to talk to who was on HD Voice on AT&T yet where as on T-Mobile almost all my friends on iPhone's or Samsung Galaxy's seem to be in HD land already.
I also contacted Verizon and added HD-Voice, which is a free upgrade and their rep claimed that HD Voice is interoperable now between carriers deploying it. That's something that needs to be proven out. If so, then services like SWITCH and UBERConference, Calliflower, WebEx and GoToMeeting all need to add the same HD-Voice codecs so when people dial in, vs. App In to the calls, they sound pristine. I just wonder how long it will take for that day to come.