David,
I too have a number that I ported over to gv that ages old (almost 2 decades), gets lots of spam, and the occasional legit caller. The number is configured on my pbx with a lengthy, ridiculous 90 second announcement before it goes to voice mail. It's been working wonders for telemarketers/robo calls. For the legit callers, they can either leave a message, or send a text.
Basically, once gv hands off the call to the pbx, gv is entirely out of the picture.