Avaya phones get locked in discovery mode

Consider the following. Office 1 that hosts the IP Telephony server have no problems. Office 2 are experiencing problems with their Avaya phones getting locked in Discovery mode every now and then.

We don't manage the telephony server but the technicians who do can't find anything wrong with it.

The two offices have Zyxel Zywall firewalls and there's a ipsec tunnel between the offices where the telephony traffic is routed.

The problem comes and goes with different phones where a phone can suddenly start or stop working.

We have seen packages being dropped in the firewall logs but we can't find anything wrong with the firewalls in question. We have even switched the office 2 firewall for a new one.

A collegue of mine tried some Telnet tests from office 2 to the telephony server and could only get through every now and then. It seems the Telnet call to the server goes through the tunnel but the reply from the server to the computer he used gets lost and generates a package dropped entry in the firewall log.

All other data that is routed through the tunnel seems to work without error.

We're really stumped here. Any help on what to try to do to solve this would be very appreciated.

If any clarification of the problem is needed let me know. This is an urgent issue!

Kind regards
Alex