That happened on various (different) phones of mine through my life.
Consider doing a ''factory-reset state'' formatting, this could solve not only this issue, but make your phone faster (it "refreshes" the phone), but take precaution with your data ('''remove the SD card!''') before that and copy all your contacts to your email provider account (meaning, do not have only one copy of the contact, as '''the formatting will erase your personal data on the phone'''), write them on an office excel sheet or paper if necessary. Also, '''this would erase all your downloaded and installed apps''' that didn't come on-default with your phone.
Hope it serves you
Narmith