We can't block an entire area code on the iPhone due to restrictions from Apple, but we can warn or block an area code and prefix.
If you receive a lot of spam calls from a specific area code and prefix, you have the ability to warn or block that in the Hiya app. There is a way to warn or block spoofed calls in the iPhone app. In the Hiya app, tap on the Blocking tab, and choose Block List. Within Block List, tap the + sign on the bottom right. You will have an option called "Numbers that begin with." You can add area code and prefixes here.
You can set a warning for up to 25 of these number combinations on the free version of the app. Auto blocking these is only available on the Premium version.
If you have a subscription to Premium you can block up to 25 of these number combinations.
You can remove an area code and prefix by sliding the number to the left and then tapping Delete.