The MWI notification on the Cisco 79x1 phones is broken since SIP software version 8.0.2. Below is the phone debug (via the console) and packet dumps of both working and broken NOTIFY requests that cause the MWI to work, or produce an Error 400 - Bad Request. Note that BOTH of the below packets are RFC3842 compliant. These errors continue in the latest software available (8.2(2)SR1).

phone debug:
DBG 22:50:18.652101 JVM: SIPProcessUDPMessage: recv UDP message from <10.0.1.24>:<5060>, length=<554>, message=
DBG 22:50:18.652981 JVM: NOTIFY sip:8080@10.0.1.37:5060;transport=udp SIP/2.0
Via: SIP/2.0/UDP 10.0.1.24:5060;branch=z9hG4bK62c3e628;rport
From: "asterisk" <sip:asterisk@10.0.1.24>;tag=as12786ddb
To: <sip:8080@10.0.1.37:5060;transport=udp>
Contact: <sip:asterisk@10.0.1.24>
Call-ID: 6cc026b778c714f92e22a28b4cd5f949@10.0.1.24
CSeq: 102 NOTIFY
User-Agent: Asterisk PBX
Max-Forwards: 70
Event: message-summary
Content-Type: application/simple-message-summary
Content-Length: 89

Messages-Waiting: no
Message-Account: sip:asterisk@10.0.1.24
Voice-Message: 0/0 (0/0)
DBG 22:50:18.656288 JVM: SIPTaskProcessSIPMessage: Received SIP message NOTIFY.
DBG 22:50:18.657539 JVM: CFGID 17: get str: line1_name = 8080
DBG 22:50:18.658406 JVM: CFGID 9: Get Val: line1_featureID = 9
DBG 22:50:18.659291 JVM: CFGID 17: get str: line1_name = 8080
DBG 22:50:18.660362 JVM: CFGID 49: get str: line1_contact = 8080
DBG 22:50:18.661475 JVM: SIPTaskProcessSIPNotify: NOTIFY: mwi
ERR 22:50:18.662408 JVM: SIPTaskProcessSIPNotify: Error: Bad MWI NOTIFY!
DBG 22:50:18.663585 JVM: sipSPISendErrorResponse: Sending response 400...
DBG 22:50:18.665749 JVM: sipRelDevCoupledMessageStore: Storing for reTx (cseq=102, method=NOTIFY, to_tag=<>)
DBG 22:50:18.667234 JVM: CFGID 195: Get Val: natEnabled = 0
DBG 22:50:18.668487 JVM: CFGID 207: Get Val: dscpForCm2Dvce = 96
DBG 22:50:18.669494 JVM: sipTransportSendMessage: Opened a one-time UDP send channel to server <10.0.1.24>:<5060>, handle = 32 local port= 0
DBG 22:50:18.670940 JVM: sipTransportSendMessage:Sent SIP message to <10.0.1.24>:<5060>, handle=<32>, length=<340>, message=
DBG 22:50:18.671814 JVM: SIP/2.0 400 Bad Request
Via: SIP/2.0/UDP 10.0.1.24:5060;branch=z9hG4bK62c3e628;rport
From: "asterisk" <sip:asterisk@10.0.1.24>;tag=as12786ddb
To: <sip:8080@10.0.1.37:5060;transport=udp>
Call-ID: 6cc026b778c714f92e22a28b4cd5f949@10.0.1.24
Date: Wed, 13 Dec 2006 GMT
Warning: 399 Bad MWI NOTIFY
CSeq: 102 NOTIFY
Content-Length: 0




Data packets:
MWI message with sip 8-2-1 Notify message that works
-------------------------------------------------------
NOTIFY sip:103@192.168.0.57:5060;transport=udp SIP/2.0
From: "Unknown" <sip:Unknown@192.168.0.5>;tag=as78f97f54
To: <sip:103@192.168.0.57:5060;transport=udp>
Contact: <sip:Unknown@192.168.0.5>
Call-ID: 5ef374d630942e2f0ac5e9121ccefb76@192.168.0.5
CSeq: 102 NOTIFY
User-Agent: Asterisk PBX
Max-Forwards: 70
Event: message-summary
Content-Type: application/simple-message-summary

Messages-Waiting: yes
Message-Account: sip:asterisk@192.168.0.5
Voice-Message: 1/0



None working MWI message with sip 8-2-1
-------------------------------------------
NOTIFY sip:103@192.168.0.57:5060;transport=udp SIP/2.0
From: "Unknown" <sip:Unknown@192.168.0.5>;tag=as78f97f54
To: <sip:103@192.168.0.57:5060;transport=udp>
Contact: <sip:Unknown@192.168.0.5>
Call-ID: 5ef374d630942e2f0ac5e9121ccefb76@192.168.0.5
CSeq: 102 NOTIFY
User-Agent: Asterisk PBX
Max-Forwards: 70
Event: message-summary
Content-Type: application/simple-message-summary
Content-Length: 92

Messages-Waiting: yes
Message-Account: sip:asterisk@192.168.0.5
Voice-Message: 1/0 (0/0)


The numbers in Voice-Message: brackets indicate (<urgent new messages>/<urgent old messages>). Although RFC3842 states that the (x/x) is optional, the Cisco software should not return an error 400 - Bad Request on receipt of a valid RFC compliant NOTIFY message.