Google Voice doesn't support the voicemail message waiting stutter tone, nor the flashing light. You can go into Google Voice settings and have it send you and email when you get a voicemail message.
To check your voicemail, simply dial your own Google Voice number from the phone. If you wish, you can program the voicemail button on the phone to dial that number. Go to your OBiTALK dashboard. Click on the 2162. Get into Expert Configurtion mode. Go to IP Phone-->Programmable keys, and scroll down to Key 7. Remove the check marks from BOTH boxes to the far right of the values, and set the key to behave as a speed dial, and enter your Google Voice phone number. Scroll down and submit.