Unknown calls

One of the best things you can do is detailed in this article: https://support.apple.com/en-us/HT207099

The section called "Turn on Silence Unknown Callers" is very helpful because your iPhone simply won't ring when there are unknown calls coming in.

There are also "spam blocking" apps in the Apple App Store. Sadly I don't have a recommendation there, you'll just have to read reviews to see what people think is worthwhile.

Good luck! 😸

Fix what?

Caller ID is a carrier feature.

If ALL of the calls you're getting are 'unknown', you need to contact your cellular carrier.

If you're just frustrated with the number of anonymous calls you're getting, look at Settings>Phone>Silence Unknown Callers and / or contact your government representative.

What do you mean by fix? If you are referring to blocking them, it really isn't possible on the phones. You need to contact your carrier and see if they provide any blocking services for spam calls.