iPhone 8 Plus not saving incoming phone numbers

for the past year or more, my iPhone 8 Plus only on rare occasions documents/saves/records/preserves/remembers the occurrence of incoming phone calls. most are completely forgotten as soon as the call has ended, so there is no way for me to retrieve the phone number or to call the person back if they're not already in my contacts list. the number appears on my screen when the phone is ringing and the last time I received a call, I asked Siri to take a screenshot to save the number as a picture, but even that failed! why is this happening and what's the solution?