Mine

[vmail-no-pass-*99]
exten => _*99,1,Set(VMBX=${SIPPEER(${CALLERIDNUM}:mailbox)})
exten => _*99,2,GotoIf($["MEEP${VMBX}" = "MEEP"]?3:4)
exten => _*99,3,Set(VMBX=${CALLERIDNUM})
exten => _*99,4,Ringing
exten => _*99,5,Wait(2)
exten => _*99,6,NoOp("Going to voicemail ${VMBX} from ${CALLERIDNUM}")
exten => _*99,7,VoicemailMain,s${VMBX}
exten => _*99,8,Hangup