Hi! I didn't look at the trace yet but have you seen my comment about the SDP file? <000b> mgcp_protocol.c:830 Got media info via SDP: port 5502, payload 112 (GSM), duration 20, addr 192.168.17.100 <000b> mgcp_transcode.c:170 Checking transcoding: GSM (3) -> GSM (112) Why would one use a dynamic payload type for GSM FR1? I am not sure if the mgcp_mgw is optimized to avoid doing transcoding if input and output codec are the same but use different payload types. I would think it is. The 0x23 UDP payload is to establish a NAT state to be able to send traffic (e.g. ringtone) to a call for a BSC behind a NAT (all traffic will go from network->NAT/Firewall->MGCP MGW) and the only way to know the port is to have the BSC send some data. So in your trace I don't see: * UDP traffic from BTS to bsc_mgcp_mgw * UDP traffic from bsc_mgcp_mgw to 192.168.17.100:5500 * UDP traffic from 192.168.17.100:5500 to ... you get the point. Maybe start looking for where the BTS is sending the traffic to and take it from there? holger