asterisk users - Sep 2006 - QuadBRI and Zyxel Wifi phone stop working together after 3 calls

Hi,
I have a strange problem that I have no idea how to debug:

I have a Zyxel Prestige 2000W Wifi telephone that is connected to my  
Asterisk server which has a Junghanns.net QuadBRI card. I can make  
exactly 3 calls to the "outside" over the QuadBRI. Any calls after  
that fail with the log saying that all lines are busy.

Turning the phone off and on solves the problem and I can make 3  
calls again before it repeats. This problem does not occur when I  
make calls from my Cisco 7960G phones using SCCP or using eyebeam and  
SIP. Also making calls from the Zyxel through a cheap Cologne chipset  
ISDN card using zaphfc does not show this problem.

I am using the following versions:
Asterisk 1.2.9.1-BRIstuffed-0.3.0-PRE-1r
Zyxel Prestige 2000W (version 1)
Zyxel-Firmware: Wj.00.11


Any help is very much appreciated,

Frederik