Appendix A: NAT router requirements for NAT Traversal feature Page 853 of 910
IP Line Description, Installation and Maintenance
Confirm using IP Phone
If an IP Phone behind a NAT device has a two-way speech path immediately
after being registered, and continues to have a two-way speech path 2 – 30
minutes later, then the NAT device’s address/port mapping is being kept alive
by the packets. If a one-way speech path occurs after this time period, ensure
that the IP Phone has the latest CS 1000 Release 4.5 firmware version. If the
IP Phone has the latest firmware, then the problem lies with the NAT device.
Try replacing the NAT device with a different model.
If the one-way speech path problem is fixed after rebooting the IP Phone, it
is likely that the NAT device does not meet the requirement of unidirectional
packet flow. As well, ensure that the firmware on the IP Phone has the latest
CS 1000 Release 4.5 firmware version (older firmware versions can be a
possible source of the one-way speech path problem).

Firmware versions

The IP Phone must have the correct minimum firmware version loaded.
Confirm using natcheck
The natcheck tool cannot be used to determine the IP Phone firmware version.
Confirm using IP Phone
On the IP Phone, go to the Services > Telephone Options > Set Info menu
and scroll down to the FW Version menu item. The minimum firmware
versions (based on the vintage of the IP Phone) that support the NAT
Traversal feature are:
• IP Phone 2002 and IP Phone 2004: xxxxB64
• Phase II IP Phone 2002 and IP Phone 2004: xxxxD41
• IP Softphone 2050: xxxx375
Earlier firmware versions do not correctly support the NAT Traversal feature.