Why does my iPhone’s FaceTime icon have a Question Mark?

When I place a cellular phone call to another another iPhone, my iPhone 13 Pro Max displays the outgoing calls information along with a FaceTime icon with a question mark within it. Sometimes it will show a call to another iPhone with a solid-filled icon. But 9.5 times out of 10 it does not have it filled solid. This happens when I call the same number at different points. So calling the same number twice will yield a different icon, usually with a question mark. Why is that and how do I fix? Or is there anything to fix? I’ll add that this happens even if calling I’m calling a family member who only has an iPhone. Any helpful comments or suggestions appreciated.