First check:
Phone -> Settings -> Call forwarding
and make sure your Google Voice number is listed under the 3 conditions. This is what sends the calls to GV. If your carrier number is there, try using the GV settings page on the web to turn it off, then turn it back on. If it still does not work, check with your carrier that "Call Forwarding" is supported on your plan. Many Pre-Paid plans do not support this.
Also check:
Phone -> Settings -> Voicemail
This is the number your phone calls when you press and hold 1 to dial your voicemail.