asterisk users - Oct 2004 - strange problem with a NT1 connected to * and an ISDN modem for data connection

Hi everybody,
I have an euro-isdn bri line with it's own NT1. I connect to the NT1 my 
* box  using a cologne chip pci card ( I run bri-stuff RC4a) and another 
pc ( a linux box used as proxy, firewall etc.).
All seems to work fine, but when I'm surfing the internet  during a 
telephone call ( in such situations both boxes are connected and  both 
NT1 b-channels are  busy)  if someone calls me he doesn't  get the busy 
signal, and my other phones ring !! Of course if I try to answer, the 
call dies (both b-channels are busy).
How can I get a correct busy signal is such situations ??
Any help would be really appreciated.
Thanks

maxx