MailboxExists( ) Conditionally branches if the specified voicemail box exists
MailboxExists(mailbox[@context])
Conditionally branches to priority n+101 (where n is the current priority) if the voicemail box specified by the mailbox argument exists. You may pass a voicemail context if the mailbox is not in the default voicemail context.
exten => 123,1,Answer( )
exten => 123,2,MailboxExists(123@default)
exten => 123,3,Playback(im-sorry)
exten => 123,103,Voicemail(u123)