I fill up my voicemail ever week, 99% of them from the area code of my phone, which is 1500 mi from where I live now, and all are spam. I wish I could just block an area code.
I use Calls Blacklist [1] to block calls from unrecognized numbers -- just checked, it supports blocking numbers based on "Starts with" so that would probably work, if you're on Android.
[1] https://play.google.com/store/apps/details?id=com.vladlee.ea...