asterisk users - Mar 2004 - support for rfc3326 The Reason Header Field for SIP

A minor gripe with our current system (* + CP7960s) is calls answered by
one handset showing as missed on others.  RFC3326 seems to answer this
problem but I see no support for it in the cisco phones or * (obviously
less of a problem with the latter being OSS).  Are cisco likely to add this?
Do any other SIP phones support RFC3326?  Anyone else have this gripe?