er .... is it just me ?
My (Windows) phone does this *already*. Without Facebook. Not that I was that impressed with the feature seeing as all my phones going back to at least 2009 (Nokia 5800) have also had this feature.
Maybe Facebook could work on a "feature" so that if the person being called is busy, the person calling hears some sort of "busy tone" to let them know to try again later.
Biting the hand that feeds IT
