Page 852 of 910 Appendix A: NAT router requirements for NAT Traversal feature
553-3001-365 Standard 4.00 August 2005
Confirm using natcheck
Run the natcheck program. Look for the message:
UDP loopback translation: YES (GOOD for peer-to-peer)
If this messages prints, then a two-way speech path should be available
between two IP Phones behind the NAT device. See Figure 274.
Figure 274
Speech path
Confirm using IP Phone
Connect two IP Phones to the same NAT device and call from one phone to
the other. Confirm that a two-way speech path is achieved during the call.

Unidirectional packet flow

Confirm using natcheck
The natcheck tool cannot be used to determine if the private-to-public
mapping created by the NAT device is being kept alive by a unidirectional
packet flow.