affecting AT&T subscribers since yesterday, regardless of their iOS or Android affiliation, is seemingly abating, but the issue seems to stem from voice-over-LTE identifiers, so changing a setting on your iPhone might help.
VoLTE for the time being. On your iPhone, go to Settings>Cellular>Cellular Data Options>Enable LTE, and set it to "Data only," instead of the "Voice&data" option you see in Apple's picture here.No, we aren't talking about AT&T's advice to keep restarting your phone until it actually begins connecting your calls, but to turn off
That's it, your calls will go via the regular voice network now, and it's prayer time that you don't get any more dropped calls because of this issue. Once you've learned that AT&T has resolved its VoLTE problems, you can return the mark back to normal, and enjoy the supposedly crystal clear voice quality that the HD Voice option offers.