CUCM with NAT after GW - first registration attempt failed with 503 but second is OK

[NikitaOst]

Hi all!

I'm quite rookie in VOIP. Hope you'll help to understand my issue.

I have CUCM and cisco phones. These phones have their static internal IPs and are behind GW/Firewall, the call manager is on external IP and sits in external zone.

All phones when go outside they passing hide NAT of the GW (the phones are registered on CM with the same IP).

My problem is next: 

After first registration on CM (always successful) when a phone sends keep-alive packets again it receives: Status: 503 service unavailable with warning 399 : "missing contact socket binding", retry-after: 30 sec.

When phone retries after 30 seconds the registration passes successfully till next keep-alive.

And it's on and on and on every time: keep-alive fails, retry succeeds.

What could be a problem of this issue?

Also I know that it worked for me earlier but one day it stopped(((

Thank you all in advance!!!

Will be very appreciated on any help!!!

[Dennis Mink]

If it worked previously, what config changed that broke it?

[NikitaOst]

The problem started only when we moved office. Our server room stayed as it was before w/o any changes in topology.