Thanks.
As long as 4 boxes are Linux it makes life easier.
I hope you know IP addresses of your asterisk.
I will use your picture:
Asteris 1 ------- FWA ---------- FWB -x--------- Asterisk 2
Please, on FWA execute command:
tcpdump -nnt src <IP of the Asterisk 1> -i <FWA INPUT interface>
You will see what asterisk #1 sends to FWA. Then you can execute it on FWB and compare those outputs.
It gives you an idea of what comes to FWA (without any changes by firewall) and what FWB receives.

Then execute that command on Asterisk 2 you will see what comes through FWB.

Try, it may give you many interesting details.

P.S. I added input interface, because FW has 2 interfaces, where tcpdump can catch source IP, so you need to specify one you need.

Thanks nimnull22,

I did what you told me about tcpdump and the packet was disappearing on the last interfase of the last fw. No packet droped at all according to the logs.

But more important is... the problem was extrangely gone now. I don't know why. I did not do anything and I tend to think people in charge of asterisk servers changed something on these boxes, although they say no.

It is maybe too soon to claim victory, but thanks a lot for your help, and I will post again if the problem returns.