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