It sounds like you've made some sort of configuration error.
First: is this a full-function Google Voice account, with a phone number that people can call to reach you, or is it a Google Voice "Lite" account, that is only used for voicemail? To be sure, log onto your Google Voice account on a laptop or desktop computer's web browser, not on your phone, and look here:
https://www.google.com/voice/#inboxOn the right side, do you see the exact two words "Your number", and underneath it is the number people can call to reach you on your OBi and on your forwarding mobile phone, or, does it say "Access number" or "Get a Voice number"?
Second: if you do have a full-function GV account, then click the gear icon and go to the Phones tab of Settings. Click the Edit button for your mobile phone number, then click "Show advanced settings." Scroll down and make sure that "Forwarding options" is set to "Go straight to voicemail." Click the Save button.
Third: Which mobile phone carrier are you using? Are you a direct customer of one of the "Big Four": AT&T Wireless, Sprint, T-Mobile or Verizon Wireless? Or, are you using a subsidiary or reseller?