Communications between telephones, server and clients

The following diagram will show the course of communications between the telephone, the server and a connected client based on ECSTA for SIP Phones in a heavily simplified manner.

Abstract description of the message flows:

After the telephone has been authenticated on the server through the SIP register, the server will create an SIP dialog for CSTA communications with the telephone. The server will register for messages about configuration changes through this SIP dialog and determine the currently configured identities. The SIP dialog will stay open continuously.

As soon as a TAPI application wants to monitor and control a telephone registered with the server, the client will be registered with uaCSTA Server for SIP Phones. The line will be open at the same time on the server. The server will asynchronously open the CSTA monitor point in the telephone and report as soon as the telephone is being monitored through a BackInService event. The client will then query all additional information relevant for it (call forwarding configuration, DND status, message waiting, etc.)
All line control functions will be called synchronously. A MakeCall message will be sent to the associated telephone over uaCSTA Server for SIP Phones and returned after successful execution. CallControl events will be sent in the opposite direction from the telephone through the server to all associated clients. Closing the line in the client will delete the monitoring point in the telephone, insofar as additional interest is not reported.