Unlike a proxy that forwards on the requests, a B2BUA acts as both a user agent server (UAS) and a user agent client (UAC) to further participate in the SIP transaction.

A user agent client (UAC) is formally defined in SIP as an endpoint that can create and manage SIP sessions.

A third option, rather than using client side XSLT, is to detect the user agent at the server and to provide different content depending on the user agent's capabilities.