Mitel/Astra phone UDP and NAT issue

Hi, At a customer site behind NAT we have three brands of phones: Mitel, CISCO, and Yealink. Cisco and Yealink work like a charm. If the Mitel phone is set to UDP it does not respond to the "Not Authorized" response to its "Registration", presumably because it did not make it back through NAT. Using TCP it works. Any ideas for NAT support with Mitel? Regards, Sean Devoy VP Operations and Development Business Focused Internet Systems, Inc.