I don't speak for Martin, but the MWI protocol is detailed in RFC-3842. The SIP messages can be easily monitored using a packet monitoring program such as Wireshark.
When a message arrives Voxalot sends a sip notify message with body as follows:
Messages-Waiting: yes
Message-Account: sip:*500@voxalot.com
Voice-Message: 1/0
The SIP phone will reply with the usual "SIP/2.0 200 OK".
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