exten => 100,1,Macro(stdexten,SIP/100,100) ; Dials SIP/100 and if busy/noanswer then leave vm in box 100
exten => 101,1,Macro(stdexten,SIP/101,NONE) ; Dials SIP/100 and if busy/noanswer then hangup

[macro-stdexten]

exten => s,1,Dial(${ARG1},15)
exten => s,2,Goto(s-${DIALSTATUS},1)

exten => s-BUSY,1,GotoIf($[${ARG2}=NONE]?3)
exten => s-BUSY,2,Voicemail(b${ARG2})
exten => s-BUSY,3,Hangup

exten => s-NOANSWER,1,GotoIf($[${ARG2}=NONE]?3)
exten => s-NOANSWER,2,Voicemail(u${ARG2})
exten => s-NOANSWER,3,Hangup

exten => _s-.,1,Goto(s-NOANSWER,1)

exten => a,1,VoicemailMain(${ARG1})
exten => a,2,Hangup