A few notes on the "Monica property": as RFC3323 suggests, I think that some
sort of anonymization service, which could be instantiated by a B2BUA, is an
appropriate way to conceal the identity of a called or calling party. What
scares me a little is the way people suggest that retargeting, as opposed to
redirection, can inherently be expected to provide privacy. When I register
a contact, I can't always guarantee that the location service in question
will be used exclusively for retargeting rather than redirection; moreover,
the revelation of the target Request-URI to the caller is not the only way
that the caller can learn who the connected-party is: contact headers of new
requests in the backwards direction of the dialog, SDP in a 200 OK, and so
on, are information leaks that are not addressed at all by retargeting, but
would be addressed by an anonymization service.
RFC3323 is about due for an update, I think, because the major functions
that it provides can be performed without a B2BUA, thanks to new SIP
mechanisms like GRUUs and session-policy. So while I agree that an
anonymization service is the right approach to this problem, these days I
think you can probably build an anonymization service without building a
B2BUA.