asterisk users - Jun 2010 - calls dropped after 20 seconds in a non NAT situation

Hi all,

I have a rather old 1.4 installation that recently was connected to a new 
network via an IPSEC tunnel. No NAT-ing is involved anywhere (I've seen
posts
about the same phenomenon but with NAT). It first the phones on the PBX 
network did not get the audio of the phones on the remote network, but that 
was fixed by removing an externip entry out of sip.conf.

What I have now, is that all calls are cut after 20 seconds. 

The log files say:

Maximum retries exceeded on transmission 629be818-f372199a at 192.168.10.104 for
seqno 101 (Critical Response)


It seems Asterisk gives up after 20 seconds to wait for some answer from the 
phone. What do you guys think is the way to proceed here: change the configs 
somewhere or upgrade Asterisk?

Thank you,

Bart