asterisk users - Apr 2009 - Asterisk and Shoretel integration

Hello everybody.

I have a problem with an integration between an Asterisk (1.4.24.1) on 
FreeBSD 7.0 and a Shoretel 7.5 server.

To make a very long story short, when someone behind asterisk call an 
extension behing shoretel everything work as expected. When someone 
behing the shoretel server call someone behind asterisk the first 10 
seconds of the call seems ok but then the line is dropped with asterisk 
reporting something like this:

[Apr 20 18:01:46] WARNING[17362] chan_sip.c: Maximum retries exceeded on 
transmission 3c2fcee15573-s9h4b1eg7wgn at snom320-0004132C5819 for seqno 
263 (Non-critical Request) -- See doc/sip-retransmit.txt.
[Apr 20 18:01:57] WARNING[17362] chan_sip.c: Hanging up call 
3998455968-113858311 - no reply to our critical packet (see 
doc/sip-retransmit.txt).
[Apr 20 18:04:52] WARNING[17362] chan_sip.c: Hanging up call 
4173967968-113858329 - no reply to our critical packet (see 
doc/sip-retransmit.txt).

The telephones behind asterisk are set up for non REINVITE... everything 
is in the same network, sip phones, asterisk server, shoretel server and 
shoretel phones.

The problem rises in the same way even when people with the shoretel 
telephone call the conference on the asterisk server, so we can cut out 
the sip phones from the general picture.

I did some SIP debugging and this is basically the problem...:


Reliably Transmitting (no NAT) to 192.168.235.6:5060:
Retransmitting #1 (no NAT) to 192.168.235.6:5060:
Retransmitting #2 (no NAT) to 192.168.235.6:5060:
Retransmitting #3 (no NAT) to 192.168.235.6:5060:
Retransmitting #4 (no NAT) to 192.168.235.6:5060:


I did various amount of empirical testing like forcing nat, setting the 
shoretel peer as insecure, turning off the light, opening the window, 
whatever.

I also did some tcp-dumping and everything seems pretty standard.

I have some (limited) access to the shoretel server and as long as I can 
say everything's ok, but I can't do tcpdumping or such there.

Any consistent idea?

Any help is appreciated.

p.s.:
shoretel sucks.