I am experiencing something similar to what you are with my iPhone 15 Pro Max. Incoming calls do not ring, however, everything else works normally. When I realize that this is happening, then I make one outgoing call and I then get incoming calls … until such time that I do not … and then I make another outgoing call … and it works again.
I have spoken to the carrier (T-Mobile) and they have been looking for a resolve for the past few weeks. What I have also found is that this generally happens after my iPhone switches from Wi-Fi to Cellular and Vice Versa. I left my phone on Cellular continuously and incoming calls rang every time for a few days until I moved to Wi-Fi and then I had to make an outgoing call to get things working again.
I believe that the iPhone is having difficulty registering to the carrier’s network every time it makes a switch from Wi-Fi to Cellular or from Cellular to Wi-Fi until an outgoing call is made (or in your case, it is a restart).
Have not found a solution yet, however am losing patience fast.
Axel F.