I used a bulk number blocking app to do exactly that, and the amount of spam calls I receive has dropped dramatically. You can also have it make exceptions for

I used a bulk number blocking app to do exactly that, and the amount of spam calls I receive has dropped dramatically. You can also have it make exceptions for numbers in your contacts list. The one I used on IOS is called Wideprotect, but I'm sure there are ways to do it on Android as well.