Hi! I have tried this with two diferents UA.... ATA -186 version 2.15 and Sipura/SPA2000-2.0.10 and always the same result.... but one thing i want to point out: the UA is not sending the ACK message only when it is located behind NAT and Ser/rtp proxy are behind another NAT......if SER/Rtp Box have real ip, no behind NAT then the user Agent is performing well and the call get two-way audio.... How can't this change it's behavior....??? always the UA is behind NAT... Always we're using rtpProxy....we modified the nathelper to include Advertise address of the rtpProxy when we located SER/rtpProxy behind Nat... and we can make successful calls from and to External customers with realip both.... but if one is behind nat....then we get just one way audio, because the UA behind nat is not sending ACK message... Any clue???? Help! Please... Thanks! Armando Cti, Miami, Fl