asterisk users - Nov 2004 - SIP phones disconnect frequently

Hello all,

I'm new to the list, but use VoIP and * for a little while now.

Running Asterisk 1.0.2 on debian linux I'm facing the following problem:

I've got two Fritz!Box Fon Adapters (kind of ATA's) with two hardware
phone
connectors each. So I'm trying to set up a PBX with four internal (SIP)
phones.
One box has fon1+2, the other fon3+4.

When I start up *, everything looks (and works) fine:

fon4/fon4        192.168.178.254  D          255.255.255.255  5060     OK
(12 ms) 
fon3/fon3        192.168.178.254  D          255.255.255.255  5060     OK
(12 ms) 
fon2/fon2        192.168.178.1    D          255.255.255.255  5060     OK
(12 ms) 
fon1/fon1        192.168.178.1    D          255.255.255.255  5060     OK
(13 ms)

But after a while (5-10 minutes), one of the boxes 'disconnects'
spontaneously:

fon4/fon4        192.168.178.254  D   N      255.255.255.255  5060     OK
(12 ms)
fon3/fon3        192.168.178.254  D   N      255.255.255.255  5060     OK
(12 ms)
fon2/fon2        (Unspecified)    D   N      255.255.255.255  0
UNKNOWN   
fon1/fon1        (Unspecified)    D   N      255.255.255.255  0
UNKNOWN   

In this situation, I cannot make calls to fon1+2. I can, however, make calls
_from_ fon1+2, to the other internal phones as well as to outside line.

Then, after a few minutes again, the phones reconnect themselves:

    -- Registered SIP 'fon2' at 192.168.178.1 port 5060 expires 1200
    -- Registered SIP 'fon1' at 192.168.178.1 port 5060 expires 1200

The same can happen do the other two phones (3+4). 

I've tried lots of different things: nat=yes/no, qualify=yes/no etc.
I've
also
switched off one of each phones, to have unique IP #s. But nothing helped.

Any ideas?

Udo