There's currently an issue with Fairphone's operating system which means that if you're using a Fairphone 3 or 4 you sometimes might notice that your incoming calls show as an unknown number instead of your contact's name. You can follow these steps to get your caller ID showing again on your Fairphone:
Go to Settings app
Click Network and Internet
Go to Mobile Network
Turn off Enhanced 4G Mode or 4G Calling
Ask a contact to call you and see whether you're able to see their Caller ID