How to block spam calls in my iPhone?

There are third-party apps on App Store can do the work, they won't block spam calls but shows a message on the incoming call screen.

There is another way to just silence unknown calls, you can do this by going to Settings App - Phone.

https://support.apple.com/en-us/HT207099

Your carrier may also help you to block spam calls, you may contact them also.

You can use Settings>Phone>Silence Unknown Callers. You can try apps such as NoMoRoBo. Also, contact your carrier and see what services they offer.