I don't think it is required to use google voices voicemail.
While the google voice app on the phone won't be able to setup voicemail (cause of lack of conditional call forwarding), you can activate it through google voice desktop version. As long as your carrier voicemail is disabled (I called them to disable it, and have them turn off web-guard), then when someone calls your google voice number, it will forward to your phone, but if no one answers the phone, after x seconds your google voice voice mail should pick up the call and your phone should show a missed call, and a google voice notification (for the voicemail).
I'm not sure why the OP is having problems. Lots of people have it setup this way.