Mine

[macro-callagent]
exten => s,1,GotoIf($[${GROUP_COUNT(${MACRO_EXTEN})}=0]?avail:busy)
exten => s,n(avail),Set(GROUP()=${MACRO_EXTEN})
exten => s,n,Dial(${ARG1}||t)
exten => s,n,Hangup
exten => s,n(busy),Busy

exten => 100,1,Macro(callagent|IAX2/dc@office/100)