Just a FYI. Received and set up my 3a today. My regular Pixel 3 works fine with Wifi calling.
In fact the new 3a shows up on my account as unknown device. That's the indication that AT&T doesn't have any support for it.
Don't think I will take a chance in getting the 3a then. Wi-Fi calling is a big deal for me. Hate to be somewhere like a hospital and cannot get a signal. That is where Wi-Fi calling can be clutch.