Callback extended features

1. We need to play a sound that says: "We are connecting your call, please hold" when the first leg answers. The name of the file to play has to be sent over the api when starting the call so it can be diferent for every call. The sound has to be stored in the filesystem. 2. We need to know when the call has been answered so we can change the state of the interface. 3. The possibility to hang up the call though API.