I'm experiencing issues, too, as of 1 Dec (Nexus 6P stock on Verizon). On 30 Nov I got at least one VM to Google Voice, but it seems that since then callers are getting the message that my mailbox is not set up.
After a quick search, found an article on XDA that suggested going to google.com / voice > Settings > Under "Forward calls to," click the "Deactivate Google voicemail on this phone" link > Follow instructions (dial *71 and call) > Click that same link to activate > Follow instructions to activate (dial *71<Google Voice #> and call). Calls are now forwarded to Google Voice.
Source: XDA forum post ID 6434949 (I've posted too few times to post w/ a link)