Understand the basic concepts of Google Voice and voicemail:
When someone calls your inbound Google Voice phone number, Google Voice then simultaneously forwards that call to up to six 10-digit linked/forwarding phone numbers, and to any Polycom OBiTALK device configured on that account, and to any Hangouts client that is signed into that account and has its "ring on inbound calls" setting toggled on.
Whichever one of those destinations answers first, within the approximately 25 second ring period, wins the race and grabs the call. If no destination answers after 25 seconds, Google Voice gives up and takes the message back to GV VM.
So: if one of your destinations answers before 25 seconds, it will always grab the call and, if that number isn't answered within its own ring interval, it will take the message.
Therefore, you need to set each Callcentric extension's "Ring for" interval to 30 seconds or longer, so that it won't win the race and take the message.