Mine

exten => 18478971337,1,Set(EXTEN=100)
exten => 18478971337,n,NoOp(${EXTEN})
exten => 18478971337,n,Macro(voicemail,${MATT})


[macro-voicemail]                                       ;Macros
exten => s,1,Dial(${ARG1},20,)
exten => s,2,Goto(s-${DIALSTATUS},1)
exten => s-NOANSWER,1,Voicemail(u${MACRO_EXTEN})
exten => s-NOANSWER,2,Goto(incoming,s,1)
exten => s-BUSY,1,Voicemail(b${MACRO_EXTEN})
exten => s-BUSY,2,Goto(incoming,s,1)
exten => _s-.,1,Goto(s-NOANSWER,1)