iPhone performance

IPhone 11 performance problems. Normally I would ask an Apple specialist this question but they don't appear to be available:

does anyone have any idea how I sometimes miss iPhone incoming calls? I can be sitting next to my iPhone or holding onto it and suddenly receive a message that I have missed a phone call. The call goes right to message with no sign that the Device rang. On other occasions immediately afterwards another phone call will work without a problem. This has been going on for years and I've never found a reason for it.