OBi phones (and ATAs) can make calls between each other using the Obitalk network. There should be a unique 9 digit number on a label on the underside of each phone.
Dial **9 followed by the 9 digit number of the other phone. If desired you can configure a speed dial or one of the buttons in the lower left corner of the phone to do that.
You can also use that Obitalk network to transfer calls or have a 3 way call between you, employee and other party on the Google Voice line. Hook flash to get dial tone, call the other phone, hook flash again. Stay on the line for 3-way or hangup to transfer.
See page 74 of
https://www1.obitalk.com/Downloads/dev/doc/OBiPhoneAdminGuide.pdf for more on Call Transfer and page 76 for Conference Calls.