Polycom MGC Administrator Administrator's Manual page 418

Chapter 7 - Configuring the Gateway
7-10
endpoint dials one number to access the gateway, and then enters the H.323
endpoint number (in E.164 format) using the TCS4 method.
Figure 7-6: System Configuration Using Forwarding Service
Using this method, the H.320 endpoint dials a number that is comprised of the
gateway access number, the forwarding service session identifier, the TCS4
indicator and the alias (in E.164 format) of the H.323 endpoint. For example,
9251815 (Note: On some H.320 endpoints there is a special TCS4 field where
you add the 501 dial string, beside the phone number as shown in step 1 in
Figure 7-7, "Call Flow Using Forwarding Service" where 9251815 represents
the gateway access number. The TCS4 information is sent by the endpoint
after its first channel synchronizes with the gateway.
The gateway identifies the TCS4 number 501 as the H.323 endpoint number
and sends it to the gatekeeper for address translation (step 4 in Figure 7-7).
The gatekeeper returns the IP address of the endpoint to the gateway (step 5 in
Figure 7-7), which then transfers the call to the appropriate H.323 endpoint
(step 6 in Figure 7-7).