[Serusers] Hold/Unhold doesn't work with NATed clients with rtpproxy in some case...

Hi: I had a very basic doubt with respect to the function "hold/unhold" on ser with rtpptoxy and kphone. Do nathelper and rtpproxy currently support hold/unhold function (with kphone 4.1.0) with nated clients ? Or do they have other methods to acheive that function? In my test, I put rtpproxy and ser in same pc, and two UACs are behind that pc, When I use hold/unhold, most time it works but some times it doesn't work I checked the ethereal log, it seems that the fail happens only when 1. hold function last less than 60 seconds. 2. once the re-invite's rtp port in SDP announced by client has been changed to different rtp port (different from original rtp port). So it seems rtpproxy doens't knows the ports come from both UACs have changed... and didn't forward UACs' rtp packets. Here is the question I have conclude from above description 1. While receive the re-invite message, will ser recheck the port in SDP and announce rtpproxy again? 2. Is this the limitation of Ser+rtpproxy with NATed UACs? 3. Does there has any suggestion that can make this function works? (ser+nathelper+rtpproxy with function hold/uhhold) Thanks and best regards Jimmy