RonR,
I had an inverse problem last night with call forwarding. I had the box checked in GV Settings -> Phones -> Forwards calls to: -> Home (landline). I received four collect calls from Washington DC (area code 202). The automated voice was unclear, but I hung up the phone three times. Finally, on the last call I was able to discern, "press 7 to block", and this stopped the calls . My home phone number is unlisted. Although this doesn't guarantee much, I have never received a collect call so the calls must have been via the GV number. And indeed, a stray area code 202 showed up in my Voice Mail.
In GV, I unchecked the Home forward to: box and enabled, Calls -> Do Not Disturb. I haven't had any further calls, but I am beginning to question the security of GV forwarding relative to robo calls and it is unclear how to block this sort of thing.