Someone I know had a similar problem with a Windows PC. He couldn't get his fax software to play with his ISDN modem, so in the end he stuck his old analog modem back in just for faxing.
I have no experience whatsoever with ISDN and I'm sure what you want can be accomplished if you're really determined, but I would suggest you get an external analog modem. If you'd gone to a local thrift store and bought an old modem for 10 euros, you could have been faxing months ago. Bonuses are greater flexibility, you can fax and surf the internet at the same time, and you have better hardware support from HylaFax.