Some carriers offer blocking all unknown calls from specified area codes. On Verizon that is "Neighborhood Filter", part of their "Call Filter" toolkit:

<https://www.verizon.com/support/knowledge-base-238154/>

<https://www.pcmag.com/news/verizons-neighborhood-filter-bloc...>

The way it's meant to be used is that the subscriber would block their own area code, and perhaps co-located overlays or neighbouring area codes. The limit for now is ten area codes.

Known or explicitly approved numbers are passed through.

The additional hack is that it's possible to request a number in any arbitrary area code, and spam and robocall rates vary tremendously across area codes. They're generally worst across the Deep South / Southeast (TX, OK, AR, AL, MS, TN, GA, SC, NC), and lowest in Alaska, Utah, Massachussetts, Washington, and North Dakota.

See:

"The Robocall Epidemic: Which states are hit hardest by spam calls?" (10 March 2026) <https://www.whistleout.com/CellPhones/Guides/robocall-epidem...>

"The Robocall Geography Tax: Why Your ZIP Code Determines Your Spam Reality" (23 October 2025) <https://www.karmacall.com/blog/southeast-spam-call-geography...>

I'd submitted the latter a few days ago: <https://news.ycombinator.com/item?id=49082473>.

Pick a low-spam, low-population state, request a number from one of its area codes (if not its one area code ;-), and then block all but known numbers from that area code.