Or is this something I should tackle on my own.

https://murphy101blog.wordpress.com/2013/11/03/raspberry-pi-telephone-number-blocker/

RPis are really quite easy once you start working with them and there is a ton of documentation online. I'd try it on your own.

This is trivial.
This requires your line to be a pots line. Can run asterisk if you are using VoIP.
I have run asterisk on old PC and vms for this

Can you suggest instructions for building one for VOIP?

How big of a blacklist are you wanting? I use VoIP.ms and you can easily setup a blacklist on their web portal. You can also set up a whitelist and send everyone else to voicemail. A lot of VoIP providers have this.

I just want to white list a few numbers - a dozen at the most, and kick everyone else to voicemail

I would just get a VOIP provider that lets you white list like you want and move my number to them. A lot less headache than the rasberry pi and/or asterisk route. Like I said, I use voip.ms but I know you can do this with several others including voipo, callcentric and anveo.

Also, there is a VOIP Tech Chat forum at DSLReports if you need help with whatever you decide to do.

Ward mundy has prebuilt installs for pi. nerdvittles.com

VOIP.MS looks like about what we need - if we filter our calls properly, we only use the phones about 180 minutes per year.

However, as most of us don't have the best hearing, is there a similar service which offers HD voice?