This is already here with WiFi networks and VoIP systems (e.g. Skype). You can log in to Skype from any phone and your calls will be billed to your account. You

This is already here with WiFi networks and VoIP systems (e.g. Skype). You can log in to Skype from any phone and your calls will be billed to your account. You can even log in to Skype on your friend's phone or you can choose another VoIP provider.