Polycom MGC Administrator Administrator's Manual page 444

Chapter 7 - Configuring the Gateway
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Gateway Link. The Gateway Link DID contains the Dial-in number
entered in the Network Service (ISDN) and 222 in the ISDN number
(in E.164 format) of the H.320 endpoint.
Step 2. The local gateway automatically dials the remote gateway using the
string 9042. The digits 90 represent the remote gateway (H.323)
prefix taken from the local Gateway Link. The Remote Gateway
H.323 profile ID, 42, represents the configured Session Profile (ID
E.164) parameter defined in the To H.323 Session Profile Definition
dialog box. The remote gateway sends a TCS4 request to local
gateway. The local gateway translates the E.164 string into TCS4 and
sends it to the remote gateway. The remote gateway translates the
TCS4 message into the E.164 alias and connects the call to the
remote endpoint. 222 is the Alias retrieved from the dial string and
represents the destination endpoint.
Step 3. The remote gateway dials 222 and connects the call.