I tested the SER+Mediaproxy yesterday, well it works well after i tried many times with SER+RTPProxy. Please forgive my stupidness.:(. But what I want to say is that nathelper can detect such situations when both SIP UA are behind the same NAT. If that, the SER can append old media IP and port to the tail of the SDP, the UA with NATed extentions can communicate with the other without natproxy helper. I wonder whether mediaproxy can achieve that goal if the author likes to do that. I think it is very easy for them if they had enough time. I am glad to hear your advice and responses Best Regards. SunZJ