you need to go to voice.Google.com and make sure and set up your number with the service.  The Google voice application for android is only a way to use it.  To set it up you HAVE to go to the website and set up your phone numbers with a verification code, which at that time you can set which numbers to ring, and when, and which voice mail service to use and other various settings.  Your phone number needs to be authenticated on the Google Voice Website before it'll work on an android device.  There are other services out there that will do just what you want, like Fusion Voice mail for Android (on the market), Google voice is way more powerful though and gives you the ability to use your cell phone like a voip on wifi networks and send text messages like data instead of messages, all this on top of reviving your voice mail visually.  If you only want visual voice mail, I might suggest using a different service.  Google Voice will set you up with a phone number that people can dial, called your Google voice number, at which point it will ring any phone number that is set up on your Google voice account at the same time, as you have set it respectively.  Just a suggestion however.