am attempting to negotiate the issues of NAT traversal with Windows messenger (4.7) as the SIP user agent

seems support for symmetric signaling is prerequisite to this -

documentation seems to suggest that Windows messenger (version not documented) supports symmetric signaling.

would therefore be expecting SIP data to be initiated from port 5060 - this is not the observed behaviour as i see data initiated from UDP port 1895 with the "contact" header being populated with the port number 16895

does the symmetric signaling need to be explicity enabled (and if so how) for versions of windows messenger ??