In a typical ISDN setup for an Inbound call from PSTN to an IP phone (-SCAC-)

CCM  –  GW  – PSTN

Setup <——-

Call proceeding ——->

Alerting ——->

Connect ——>

  1. Gateway and PSTN switch exchange Q921 Layer 2 information. Gateway registers with ISDN switch and terminates Q921 at gateway.
  2. A Q931 Setup message arrives. The gateway puts it in a packet and send it to CCM via TCP port 2428 (Backhaul)
  3. CCM then sends three Q.931 messages – Call proceeding, Alerting and connect
  4. After the call is brought up, CCM sends the gateway an MGCP MDCX to tell it parameters for connection
  5. Gateway sends an ack of connection

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