Hangouts integration with Google Voice was discontinued on April 1. Google announced this pending change last year. You can no longer use Hangouts to send SMS to/from your Google Voice number. You'll need to use its designated replacement, which is the Google Voice app and the Google Voice website.
May be a stupid ? but I'm never sure if there are features I'm ignorant of.
I see that the GV dialer has a "hold" button. What does it do? Can I make 3 way calls by putting call 1 on hold, make call 2 then "un-hold" call 1? I couldn't get it to work that way.