I think that is unusual for many people to receive a call from a code area that is unknown, I tried another applications and them didn’t worked in my cellphone with LinageOS, the application that worked the best for me was Mr. Number .
So, first install Mr. Number in the Play Store or use the Yalp Store if you are concerned about privacity and OpenSource.
In order to block unknown code areas numbers calls, first enable block incoming call from outside your country, and then add in your block list restrictions for numbers that begins in:
1, 2, 3, 4, 5, 6, 7, 8, 9
In except of the one of your own lada; then suppose that your own code are begins with three, then start blocking numbers begining in:
31, 32, 33, 34, 35, 36, 37, 38, 39 and 30 except your own area code, and do the same with specific code are that you want to allow, i.e, blocking all the pairs of numbers except for the ones that you one to allow.
Another posibility is to block any number that is not in your contact list using Mr. Number too, that is the easiest way.