Return code en ringing mejorado.
/* It is common practice for channel drivers to return -1 if trying
* to indicate ringing on a channel which is up. The idea is to let the
* core generate the ringing inband. However, we don't want the
* warning message about not being able to handle the specific indication
* to print nor do we want ast_indicate_data to return an "error" for this
* condition
*/
main/channel.c