Voicemail busy o u

hello forum, I have a asterisk server and my extensions sip they are of 3 digits, I want is that when a person this busy one and another extension calls him that it jumps to the voice mailbox and that he says that the extension this busy one or to the telephone leaves its message and when not this speaking that a message comes out of not available…

I have inside the dialplan this configuration, if this busy one or not this busy one always comes out not this available one

exten => _XXX,1,Voicemail(u${EXTEN})
exten => _XXX,n,Hangup

exten => _XXX,1,Voicemail(b${EXTEN})
exten => _XXX,n,Hangup