ENDIAN Firewall - Suddenly all Incoming SIP calls disconnected

hi everyone!

i need help here, i am desperate. This problem is very strange because i can't find a reason for it.

This is the problem:

I set up an endian machine, configure fixed leases in DHCP, block all other IPs and enable SIP and http proxy in transparent mode. Everything works fine.
Everybody can make and receive calls (4 SIP Phones). Suddenly though, the phones ring on incoming calls but as soon as you pick up, the caller gets disconnected while the SIP phone starts to count the time being connected to the caller. Needless to say, the connection has been terminated and you can't communicate.
Making calls still works without any problems.

Here's what I did: i reinstalled the endian machine taking one step at a time (first without fixed leases and so on) but it just keeps crashing.

I haven't found a pattern in what time it takes to happen - this problem appears irregularily and can't be fixed by rebooting the endian machine or restarting the siproxd... the only way I found to make it work again is to reinstall the endian machine. Since I have done that many times now I started to just backup the settings - this speeds up the reinstallationl process - but still, I can't reinstall every some days/weeks.

I haven't got any idea left how to solve this problem and am uncertain what info's I have to provide you with. Please help me!