Using ISDN line via router across network

I have been informed the ISDN line box, when installed, can be no farther than appx. 2metres from your router/terminal adapter. Is this the case? If so why? If so, instead of running a Cat5 cable about 20metres or so to an installed TA on PC, can I install a 3Com 531s router next to ISDN box, and then patch through a 3Com switch to any point on the network? Apologies for this long winded question.....

from the wall jack for the ISDN connection, you need to have your ISDN modem. from there you can run cat5 if needed.