[callback]
include => callback-custom
exten => 1,1,Set(CALL=514XXXXXXX)
exten => 1,n,Set(DESTINATION=ext-local.${VM_PREFIX}201.1)
exten => 1,n,Set(SLEEP=0)
exten => 1,n,System(/var/lib/asterisk/bin/callback ${CALL} ${DESTINATION} ${SLEEP} &)
exten => 1,n,Hangup