I read online that you can set the Google Voice iphone app to work on data/wifi only, ignoring the carrier, so in order to avoid roaming voice charges while away the plan was to set my TMO number to call forward to the Google Voice number, then use GV to answer the call, or make calls back, using data.

I configured the GV app for data/wifi, set call forwarding to the GV number, but when someone calls me, the GV app does not ring - the caller gets a "GV subscriber not available" message, and I see a missed call on the app.

Am I missing something or doing something wrong?