It should work by default, using automatic DTMF method selection, but you can try forcing DTMF (Touch Tone) to use the "Inband" method, as drgeoff posted above.
On the OBiTALK web portal, click your device, then get into expert configuration mode, and go to Service Providers-->ITSP Profile A General (or B, C, or D, depending on which SP you are using with Google Voice) and change DTMFMethod to InBand. You need to un-check BOTH boxes to the right of the setting to change it from the default. Click the Submit button to apply the setting change.