You are trying to turn Google Voice into something it's not. It is not a "free telephone company". It is not a bare-bones SIP ITSP. It is a complete, integrated telephone call forwarding, calling, voicemail and text messaging service. You cannot pick and choose, and you cannot make it work like an old-school SIP ITSP. You cannot disable its voicemail and use Callcentric's voicemail. IF you attempt to do that, and you are on a phone call, and another call comes in, that call will go to Google Voice's voicemail. Now, you will be worse-off, because you have VMs going to two different places.
Google Voice was not designed to be a standalone SP for OBiTALK products. It was designed to enable OBi's to just one more place where you can send calls. Google assumes you are using their service as designed, which means you have a smartphone that can run the mobile Google Voice app. You receive notifications via the app, and/or via email. I already explained that you can set it to push a notification to your phone of missed calls and/or voicemail messages and/or SMS/MMS messages. You can also have it send an email notice. Those are the notification methods, not MWI. Period.
If you don't like this, like zsak23 said, go get your VoIP service from some other company. You are just being unreasonably argumentative because you want everything for free. This service doesn't meet your needs, so go pay a modest fee to use another service and not Google Voice.