[abrigham]

You can ignore the prompt that says it's not compatible. That's just referring to the automatic activation. You can still do manual activation.

Have you already gone through the instructions on the Google Voice website to enable Google Voice voicemail for your mobile number? To double check, log into your Google Voice account from your computer, go to Settings -> Phones and click "Activate Google voicemail on this phone" and it will give you the manual setup instructions... If it says "Deactivate Google voicemail" then it's already set up.