[phones]
exten => 100,1,Dial(SIP/100)

exten => 101,1,noop(CALL FROM ${CALLERID(num)})
exten => 101,n,GotoIf(${CALENDAR_BUSY(calendar)}?busy:available)
exten => 101,n(avaliable),noop(CALENDAR: AVAILABLE)
;exten => 101,n(available),dial(SIP/101)
exten => 101,n,Hangup
exten => 101,n(busy),noop(CALENDAR: BUSY)