it will get much easier and actually this problem is the default Google Voice that is installed on the phone is missing some options. If you update the app from the market it gives you the option to "skip" the integration part and use the old google voice settings. when you hit skip your phone doesn't get taken over by Google Voice.
During the updated Google Voice set up it will give you a *28XXXXXXXXXX number to dial to have your phone forward voicemail to goovgle voice and ONLY that.
During the updated Google Voice set up it will give you a *28XXXXXXXXXX number to dial to have your phone forward voicemail to goovgle voice and ONLY that.