Google Voice's voicemail function is deeply integrated into the design of the service. If it was disabled, it would cause problems with its simultaneous ring function and the ability of linked phone numbers to forward unanswered or busy calls back to Google Voice's voicemail. Google Voice allows calls to be placed and answered on an OBiTALK device, but that is not the primary purpose of the service.
Google Voice is not a bare-bones SIP VoIP trunk provider. If that's all you want, then use a different service provider.