Assuming you are connecting it to an ISDN line, assuming that the router has a serial port with V.25 dialing (toggle the DTR line low to dial a pre-configured number -or is it the standards version of ATDT ???-), then yeah, sure, it should work.
If you're talking about hooking it up to a POTS line or if the router only has a couple Ethernet ports, then No, you probably can't do it.
What's the ISDN modem, and what's the router? What did you have in mind?
FWIW
Scott