That would suggest that you've accidentally marked that caller's phone number as spam, or you enabled blocking for that number. This has nothing to do with your OBi device.
Log into your Google Voice account from a laptop or desktop computer's web browser, not from a tablet or phone. Go to your GV History page:
https://www.google.com/voice#spamIf you see a record of a call that went to voicemail, then place a check-mark in the box to the left of the call record(s) and mark it/them as "not spam". Also, if the calls show as "blocked", click the "more" link and then click the "unblock" link.
