Open Google voice app, menu> more> settings.. The second option is making calls. Tap that and select "ask every time ". You're all set.. Whenever you make a call it'll ask you which number you want to call from.
There is a widget (comes with the Google Voice app) that lets you toggle between these four options:
Use Google Voice to make all calls
Do not use Google Voice to make any calls
Use Google Voice only for International calls Only
Ask every time a make a call