[Users] sdp negotiation fails - no ACK - rtpproxy

Hello! I'm struggling with a issue since several days, since i make no more efforts i'd like to ask you for help: I'm trying to attach a exchange 2007 server to openser, most of the features work(inbound call, refer) but i can't get a outbound call to work as exc doesn't seem to like the SDP in my "200 OK" answer, i get no ACK and no rtp-stream is sent from exchange. My setup looks like this: ________ _______ _____ ________ |asterisk|=======|openser|======|(NAT)|=======|exchange| |________| |_______| |_____| |________| 2x.x.x.107 2x.x.x.106 62.x.x.193 10.1.99.97 A rtpproxy is running on the openser too, exchange is talking tcp... In my openser-config i use the nathelper-module and rtpproxy to traverse nat for the clients. This setup is working for all phones i've tested so far, so imho it is useable. Exchange has a feature that allows dialouts to read voicemail to e.g. a mobile phone. It is using a standard INVITE dialog, in a local network without nat i've been succesfull to attach exchange to sipx and a phone registered at sipx and i guess it will also work with a local openser who doesn't use nat-traversal features. However, with the public openser where i have a NAT, exchange doesn't seem to accept my sdp-proposal, i get no ACK and the rtp stream is only sent by asterisk, not by exchange. Unfortunately exchange offers no valuable debugging information, so i have no idea what exactly it doesn't like about my answer - i did some trial and error with different nat-helper-functions, activated and deactivated them but so far it didn't make a difference(or the call didn't got through at all due to missing nat-traversal). I've uploaded wireshark-traces exported to text of the working local dialog with sipX(http://pastebin.ca/383516) and the non-working external dialog (http://pastebin.ca/383540) with openser, maybe someone can take a look at it and check if there's something wrong on my(=openser) side of the dialog - Thanks in advance! best regards Christian