[Serusers] SER + NAT (Allan, please?)

Hello, We are trying to get SIP Express Router and RTP proxy working for NAT traversal (as many of you, I think). We have tried all kind of configuration files, versions, etc. but the best result we get is always the same: the session is stablished, but the media are not. We have followed, step by step, Allan's help (thank you, I am the guy that asked for your ser.cfg) in his post on Nov 4th (http://lists.iptel.org/pipermail/serusers/2004-November/012737.html), but we are not able to get it working. The problem seems to be that SER does not send the media to the ports indicated by SDP, instead it sends them to wrong ports resulting in ICMP 'destination unreacheable' errors. As clients we are using two Windows Messenger 4.7 in two different ADSL (NATed) lines connecting to SER in a public Internet address. The linux box for SER is Red Hat 9.0 and file versions are the same as indicated in Allan's post. Any suggestions, ideas, ..., please? The deadline is really near and we do not know what else to do. Thanks in advance, Luciano Bajo