The feature you are describing is known as "click-to-call". It is no longer supported by Google Voice for OBi devices. Instead, you can add a real telephone number, and have GV call THAT number to bridge the call. For example:
Get a free or paid inbound telephone number (DID number) from a SIP VoIP service provider like Callcentric. Add and verify that number on your Google Voice account's settings page. Remove the check mark next to Google Chat, and add a check mark next to your SIP DID. Provision the Callcentric number on one of your OBi's SP slots. Now, inbound calls will route through Callcentric, which will also give your Caller ID name and (for a small fee) 911 service. You will be able to select this DID number when using GV's click-to-call function.