Common issue with new GV setups and an OBI.
Go to GV and check for unread GV VMs in: History, Spam, Saved, etc., etc.
Then check the following:
Use the latest firmware.
Check the "MessageWaiting" field in the Obi. It's somewhere under the SPx Service group. Note that it is "read only" to you a a user, so you cannot toggle it. It if does not match what the phone shows, reboot the Obi (may have to do this twice.)
Also double check whether the suspect SPx service is in fact 'pointed to' the cordless phone, X-InBoundCallRoute, should be ph for both SP1 and SP2.
Disclaimer, I don't have an Obi100, I'm going from using a 110 and a 202.