I have had the same problem before. I During setup, the Google Voice app (this is what you mean, right) asks whether you want all calls router, just international calls, or no calls routed through it. If you didn't select All calls during setup, it cannot be found in settings. But the solution is to use the Google Voice toggle widget (looks just like the Google voice all, 1x1). Place it on your home screen, then tap on it until you see the toast that says Route All Calls through Google Voice (or something like that).
Otherwise, if you want everything to be routed through Google Hangouts, you have two options. But note that this routes calls through your data plan, not your phone plan. Both solutions require use of the Google Hangouts Dialer app. Download it from the play store. The first method is simple. Replace your dialer with Google Hangouts Dialer. When you open the app, it takes you to a tab in Hangouts that contains a dialpad, or your call log. If you do not see the dial pad, click the dialpad button at the bottom. Then dial, like you normally would. The second is a little more complicated, but it allows you to use your existing dialer. Clear your app defaults for both Hangouts Dialer, and your native Dialer app. Then when you dial a phone number, your phone should prompt whether to call using your phone app, or Hangouts Dialer. Select Hangouts Dialer.
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