Thanks, @ProfTech and @azrobert!
I think I will just go with the Obi212, but thanks for the generous offer on the ObiLine ProfTech! Since the Obi212 is a pretty new box, I figure it will get longer term support.
My current thinking is to forward the ATT landline/POTS number to the GV number using ATT's forwarding service. That way, all calls can be filtered by GV. I gave it a try, and it appears the correct originating number does indeed come through (as opposed to my parents' POTS number that I am using just to forward).
I may just have all outgoing calls to through the POTS line, since I am less worried about monitoring the calls they make out vs the calls that come into them. That way, should they lose internet service, I am hoping they can still make outgoing calls regardless (??).