ISDN Link is for in-room ISDN and external network connectivity supporting Cisco TelePresence EX, MX, SX, and C Series endpoints not for MCU.
It provides direct connectivity to ISDN or external networks (V.35) for individual telepresence endpoints without the need for additional gateways or other infrastructure (i.e. Gatekeeper).
If you are looking for MCU to ISDN network, you will need ISDN Gateway, like TelePresence ISDN Gateway 3241.