- The Cisco TelePresence ISDN Link is a compact appliance for in-room ISDN and external network connectivity supporting Cisco TelePresence EX, MX, SX, and C Series endpoints. It provides direct connec tivity to ISDN or external networks (V.35) for individual telepresence endpoints without the need for additional gateways or other infrastructure. The ISDN Link helps ensure that ISDN can be used as the primary, backup, or external connectivity for a high-quality and reliable telepresence experience.