Are you sure? There are actually two (possible) dialogs:
- One is a white-themed dialog with a header labeled with the Google Voice icon + "Google Voice", the choices "Call with Google Voice" and "Call without Google Voice", and a cancel button.
- The other is a black-themed dialog with a header labeled "Call" and the choices (on my phone) the CSipSimple icon + "[my VoIP number]" and the Dialer icon + "Use Mobile".
If your phone is set up the way I think it is, when making a call from Google Voice you'd see the first, white dialog. If you choose "Call without Google Voice" it would make a normal cellular call. If you choose "Call with Google Voice" it would still make a cellular call, but the number called would be Google Voice's number itself, whereupon it would then be routed through Google's servers and then to its destination. But it would still use cellular voice protocols between you and Google. (Your phone would report that it called the number you told it to call, but if you requested a call log from your cellular provider it should show you calling Google instead.)