I had Google Voice installed, but it was a very obtrusive and annoying application. For example it one day starts stealing text messages and requires me to press a 1 to receive the call, neither of these "features" can be disabled in the Google Voice settings on the phone.
Since the only feature I ever used with GV was voicemail transcription, the cons overweighed the pros, and I figured it's time to uninstall.
I uninstalled the app, went to the google voice webpage, and disabled google voice there. Now, I am unable to receive phone calls. The caller is greeted with a busy signal.
Settings > Call > Voicemail Service = My Carrier
The google voice number was set to my actual phone number.