Taken from the online help:
The MCU can then effectively call through these configured gateways to one or more endpoints which are registered with the gateway but would not be reachable directly from the MCU.
Refer to the "Managing Gateways" section of the MCU Series Online Help starting on pg 120.
If you have any more questions, you should move your question to the TelePresence section of the forums where these devices are more actively discussed. You can move your discussion by editing your post and changing the categories at the bottom.
