Purchase a Di*it**e Call blocker.  It's wonderful in that you can block entire area codes.  Even if you have clients or friends in a blocked area code, you can "invite" their phone numbers into the blocker and they can call right through.
We have two Di*ti**e on our business lines and one on our home line.  They work wonderfully!
Currently, we have over 230 area codes blocked and each area code has the capability of 6-8 million phone numbers!

Do you recommend  any particular one and from what source?

Try Amazon - they have numerous blockers.
We have chosen to go with the Di*it*ne blocker because it can block entire area codes.
We have had the 202 area code blocked for over 2 years along with over 260 other area codes.
This may or may not work for you.
Should you have friends/family in a blocked area code simply "invite" their number into the Di*it*ne blocker - they can call right through!

Called leaving no voicemail. Blocked.

We have had the 202 area code blocked for over 2 years now.
Purchase a call blocking device that can block entire area codes as well as full telephone numbers.
Stop the telemarketer madness!