iPhone Call Fails - Calls straight to voicemail whether on cellular or WIFI calling - iOS 18.6.2

My only solution was to turn off Wi-fi calling. T-mobile investigated my call drops and said they were all wi-fi calls, so know we force cellular calls. T-mobile suggested that I might need a new router or that I should call our internet provider (Comcast), tell them about our problems with wi-fi calls, and ask them if they are blocking a port that they shouldn’t be blocking (that’s as technical as the conversation got). Because turning off wi-fi calls seems to have cured the problem and our lives are busy enough, I have not called Comcast to investigate this further. I hope you mom’s calling problems end soon. It was a pretty frustrating few days for us. Best, John

Thanks, for the trouble-shooting tip. I did not test this rigorously because turning off wi-fi calls seems to have solved our problems and I am happy to leave wi-fi calling off. Based on T-mobile’s investigation, my problem seems to be linked to my router or my home internet provider’s port settings (see my other reply below). I guess I should troubleshoot this further by confirming that I can do wi-fi calls on other networks, but I have not got around to testing that yet. Back to your question of FaceTime audio, I *think* that my office’s front desk called me once using FaceTime and it worked—in so far as the call came through and we had a short conversation. Not absolutely sure about that though, and sure what this would mean in any event: do you know if FaceTime audio calls use the same communication protocol as wi-fi calls through the phone app?

Thanks. As noted in another reply, that I just posted a minute or two ago. I called my carrier, they said things checked out on their end and suggested I contact my internet provider (or perhaps get a new router). I haven’t done that yet because just turning off Wi-Fi solved the problem for me, and I’m content to leave it there for a while. but thank you for your response.

Additional info: also reset network settings on both phones. Problem persists.