The formula for successful far-end NAT traversal is: 1. CPE with symmetric NAT capability (most CPE these days). 2. Far-end media relay and draft-comedia style media source port detection. This one is really key. It is critical for the service provider to ignore the media ports advertised in the customer-side SDP and "listen" to the media stream for the "actual" source port that is translated by the NAT gateway. This requires a media gateway and/or relay that has the intelligence to wait at least one packetisation cycle for RTP received from the customer end before sending media back to it, and does assume symmetric RTP. Most higher-end commercial SBCs can do this, but the option has to be explicitly turned on. The default behaviour here may account for the difference you see. There is pretty much no way to solve this problem without media relay at the service provider end, i.e. in case you were hoping for a purely proxy-based solution. 3. Yes, force rport. 4. Yes, aggressive override of network and transport-layer identifying information in SIP headers. 5. Disable all SIP ALGs on any firewalls and routers on the customer side. -- Alex Balashov - Principal Evariste Systems LLC 1170 Peachtree Street 12th Floor, Suite 1200 Atlanta, GA 30309 Tel: +1-678-954-0670 Fax: +1-404-961-1892 Web: http://www.evaristesys.com/