For what it's worth, ATT just DM'ed me:
Thank you for reaching out to us regarding your concern. We are aware of the issue and as this is a temporary intermittent issue that we are working hard to resolve. If you continuously experience persistent problems for a specific mobile number you are calling then try you should turning off Wi-Fi Calling before placing the call. ^RobbW
So, sounds like they're aware of the issue (thankfully), but hopefully this is revolved SOON...