Having a digital ISDN telephone connection lets you easily integrate Fax and Answering Machine functionality into your Linux server. SuSE (and Debian) distribute a package called Capisuite for this purpose. You will also need a supported ISDN card (an AVM Fritz card is inexpensive and works).
The big issue is the Linux connectivity which became worse an worse over the time as Linux distribution getting more and more complicated. As matter of fact I did not find one instruction in the Internet on how to connect it to Linux via a cradle. Here is How!