Google Voice is extremely simple to set up. If you can't or don't want to use the OBiTALK configuration facility:
System Management -> Auto Provisioning -> Auto Provisioning -> Method : Disabled
Service Providers -> ITSP Profile A -> General -> SignalingProtocol : Google Voice
Voice Services -> SP1 Service -> AuthUserName : (Google Voice username)
Voice Services -> SP1 Service -> AuthPassword : (Google Voice password)
Voice Services -> SP1 Service -> X_SkipCallScreening : (checked)
Physical Interfaces -> PHONE Port -> PrimaryLine : SP1 Service