Well, it's as easy as 1) using the OBiTALK dashboard to delete the SPx configuration you were using with Google Voice, then 2) click that SPx and, on the next page, don't select Google Voice again; instead, scroll down to the bottom of the page, click on "OBiTALK Compatible Service Providers", and, on the next page, click on Callcentric to configure the service.