exten => 1718,1,Set(GROUP()=${EXTEN})
exten => 1718,n,NoOP(Group count is ${GROUP_COUNT()}, group is ${GROUP()}, exten is ${EXTEN})
exten => 1718,n,GotoIf($[${GROUP_COUNT()} > 1]?busy:avail)
exten => 1718,n(avail,Dial(SIP/${EXTEN},20,rtT)
exten => 1718,n(busy),Busy