That's not quite all, maybe I should have posted a trace with a message waiting, but nonetheless, the message account value in the SIP Notify message contains the value that the message button uses to call the mailbox, ie it will call 841@xyz.com when I want it to call *97 (or 897@xyz.com if that will work) when the MWI is lit.
841@xyz.com doesn't work as that mailbox is disabled.