Major:
    This document defines a new code for use in SIP, and specifies new behavior both for the code itself and for its use in history-info.  I am thus confused as to how this can be an informational RFC.  It looks like it either Proposed Standard or experimental.  Yes, I see that RFC 4458, which this updates is Informational.  But just because we did it wrong before does not make that behavior correct now.  In addition to my understanding of the roles of different RFCs, I note that RFC 3969 and the IANA registry both state that this assignment must be made by a standards track RFC.

Minor:
   Given our emphasis on IPv6 over IPv4, would it not make sense for the examples to use IPv6 addresses?  (Inspired by the Id-Nits alert.)