Page 850 of 910 Appendix A: NAT router requirements for NAT Traversal feature
553-3001-365 Standard 4.00 August 2005
4The Private-to-Public mapping created by a NAT router must be kept
alive by packets in only one direction (standard in most NAT routers).
5The IP Phone must be running the correct minimum firmware version
(not a NAT device requirement, but still important).
Most of the issues encountered can be confirmed using either an IP Phone
behind the NAT device or a PC running the third-party tool “natcheck”.
Note: Nortel is not affiliated in any manner with the natcheck tool, and
therefore is not liable or responsible for any problems that may be
encountered with this tool.
The natcheck tool can be downloaded at no cost from the Internet at:
http://midcom-p2p.sourceforge.net/. The natcheck tool runs in Windows on a
PC connected to the internet through a NAT router.

Requirements

Cone NAT

The NAT Traversal feature cannot work unless the NAT device has a cone
NAT implemented.
Confirm using natcheck
Run the natcheck program. Look for the following message:
UDP consistent translation: YES (GOOD for peer-to-peer)
If YES is displayed, then the NAT router uses a cone NAT.