That is just OBi 101, as pointed out by dircom.
Sign into the correct Google/Gmail account that holds your Google Voice number, and go here:
https://voice.google.com/settingsConfirm that you see your inbound Google Voice phone number on that page; if you don't, then you are either signed into the wrong Google account, or your number was reclaimed. Assuming you see it, then...
Press Ctrl-T to open another browser window, and sign into your OBiTALK dashboard, here:
http://www.obitalk.com/obinet/pg/obhdevClick on SP1 or whichever SP you want to use. Follow the instructions on-screen to configure it to use your Google account. Make sure, when it pops up a window asking you to confirm that you are granting OBiTALK permission to access your account, that it displays the same Google/Gmail address as you saw earlier. Finish the steps. That's it.