Looking to make free calls with Google Voice? Well, You will have to use the Google Voice app over your data connection.
Google’s documentation for the Google Voice app is quite clear how it makes phone calls with your phone so you will need a third-party app like GrooVe IP, SIPdroid, etc, to use Google Voice over data with a tablet such as the Nexus 7 so that the hidden native dialler won’t try to make calls to run up your data plan and use wi-fi instead.
Please make sure “Forwards calls to Google Chat” is enabled if you want to be able to accept incoming calls on your device. You can find this feature by going to “Phones” tab. While there, make sure “Always ring on weekdays/weekends” is on.