The service is called Google Voice. In the US and Canada Google Voice is free for people using GV to GV. As for rates to mobile networks they differ according to the country that you want to call. But the rate applicable when calling to Zimbabwe are as follows:
Zimbabwe - TelOne US$0.08
Zimbabwe - Mobile - Telecel US$0.33
Zimbabwe - Mobile - Econet US$0.39
Zimbabwe - Mobile - NetOne US$0.23
Zimbabwe - Premium US$0.46
I am not sure whats covered under premium but it might be soft-phones and satellite phones.