[macro-stdexten]
exten => s,1,Dial(${ARG1},20)
exten => s,2,Voicemail(u${MACRO_EXTEN})
exten => s,3,Goto(incoming,s,1)
exten => s,102,GotoIf($[${DIALSTATUS}=CHANUNAVAIL]?103:105)
exten => s,103,Voicemail(b${MACRO_EXTEN})
exten => s,104,Goto(incoming,s,1)
exten => s,105,Voicemail(u${MACRO_EXTEN})

I call it via my extensions section 
 
[extensions]
 
exten => 120,1,Macro(stdexten,Sip/121) ; Ring for 20 seconds
exten => 121,1,Macro(stdexten,Zap/1)