Where can I get C headers for CTI driver?

Hello!

We are using Siebel 8.1 with Avaya PDS telephony. But in the nearest future we want to migrate CTI from PDS to another solution, which leads to developing our own CTI driver. There is a chapter in Siebel CTI Administration Guide about developing a CTI driver with the description of handles, methods, etc, but the question is where can we find any header files to use? Or maybe there is some library for driver development?

Thank you.