> As per RFC 3261 of section 12.1 and 14.1, it should be 
> send to abc@user same thought i have , But not mention 
> it is mendatory. Please suggest if any other RFC ?

I'm not sure if you will find anything that explicitly precludes the UAC from 
updating things.  However unless the vendor finds something definitively 
indicating that it should, the UAC might be incorrectly interpreting the reason 
why the Contact was included within the failure response.  For instance, some 
vendors have considered adding Contact (and Record-Route) to a failure response 
as a way to communicate which hop generated the failure response.

RFC 6141 attempted to fix/clarify RFC 3261 concerning target updates.

4.7.  A Response Updating the Dialog's Remote Target

   If a UA receives a reliable provisional response or a 2xx response to
   a target-refresh request, the UA MUST replace the dialog's remote
   target URI with the URI from the Contact header field in that
   response, if present.

   If a UA receives an unreliable provisional response to a target-
   refresh request, the UA MUST NOT refresh the dialog's remote target.


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