Sign into your OBiTALK dashboard:
http://www.obitalk.com/obinet/pg/obhdevLook at the list of devices. I assume you will see your OBi device. If its list of SP1-- SP4 is not shown, click "expand" at the far right. You should now see the list of SPs, and you should see your Google Voice number with a status of "Connected".
Click the device name, and it should take you to the device's main configuration page. Click "OBi Expert Configuration", and click through the various confirmations to get into expert mode. Once there, assuming Google Voice is on SP1, click "Service Providers"-->"ITSP Profile A General". Look at the parameter "DTMFMethod". It should be set to "Auto". If not, un-check BOTH check boxes to the far right and select "Auto", then scroll down and save your settings. Test the ability of Google Voice to recognize pressing 1.
If it still doesn't work, then go back to that setting and change it to "InBand", save your settings, and repeat the test. Does it work now?