Custom VoiceMailMain

How to send a SIP header variable to Activate the MWI in an extension?

I try:

exten=>27,1,Answer()
exten=>27,n,SIPAddHeader(Call-Id: 210@127.0.0.1)
exten=>27,n,SIPAddHeader(CSeq: 4 SUBSCRIBE)
exten=>27,n,SIPAddHeader(Contact: sip:210@127.0.0.1)
exten=>27,n,SIPAddHeader(Event: message-summary)
exten=>27,n,SIPAddHeader(Expires: 86400)
exten=>27,n,SIPAddHeader(Accept: application/simple-message-summary)
exten=>27,n,SIPAddHeader(Content-Length: 0)

But the Extension 210 Message Waiting Lamp don’t turn ON :S