Hi Pat:
Log into your OBiTALK portal account and click your OBi device on the list. Click the links to get into Expert Mode.
Assuming your Google Voice account is set as Service Provider #1 (SP1), click on Service Providers, then ITSP Profile A General. Find the "DTMF method" setting. Clear the two check-marks to the right of the setting, so that you can manually edit it. Change the setting to "InBand", and then submit your change.
In my screenshot below, GV is configured on SP4, so I edited Profile D. Pick your SP and edit accordingly.