The easiest way is to use the sample desktop provided in the CTIOS Toolkit and use Visual Studio VB.NET to add your own code.
You would normally extend the delivered or established event in the code to extract peripheral variables or ECC variables to send to your Web service, get back the results and parse the XML response, then launch a window showing the data you want to the agents.
Not difficult for a VB.NET coder. Just read the Programmer's Guide to see the classes and methods. Using some of the code in the Outbound application in the Toolkit as a guide to extracting PVs, ECC vars.
Regards,
Geoff