Hi,
The problem you mention is working as designed. In the RFC look for the section
Code:
4.1.2 Request-URI
The Request-URI field is a SIP URL as described in Section 2 or a
general URI. It indicates the user or service that this request is
being addressed to. Unlike the To field, the Request-URI field may
be re-written by proxies.
Another link describes it as "bad practice"
http://www.archivum.info/serusers@ip.../msg00535.html