Perhaps this is more of a hardware question and not even a linux question.
You should be able to buy a stock isdn firewall / router combination. If you have an outdated pc you could transfer it to a great linux router using
http://www.fli4l.de/e_index.htm.
I use fli4l on my home network with adsl and configured a similar system for my parents who only have isdn.
It took me about half an hour to set everything up and works perfectly. Have a look at all the great features fli4l can offer to you, I run it on a 486-33 with only a floppy installed and a isdn isa card that cost me 1€.