FaceTime Won't Connect on Calls - Sends Notification Instead

My piano teacher and I use FaceTime for lessons. In the past, he would call me with FaceTime, I'd answer, and we'd go on with the call. Now when he calls me, my phone doesn't ring, but I get a notification and have to click it, then click "Join FaceTime" to join and be able to talk to him.

This started in the past 6 months, after he upgraded to a new iPhone (my guess is it was brand new and is a 15). He's not a techie and I've looked through my FaceTime settings and can't find anything about this. It confuses him because he calls me and nothing happens for a while, until I click "Join FaceTime." It's frustrating for me because if I'm distracted when he calls, I don't always hear the notification because it's not that loud.

Which phone is causing this change and what can he or I do to make it so FaceTime calls just go straight through to me, by ringing like a phone call, and letting me answer it directly, like it used to do?