A few years ago I wrote a simple c# app which used the Traysoft Tapi classes to interface with our kx tda Panasonic PBX in our office. Only four extensions and when a phone rang and the op lifted the phone the app on the pc attached to the pbx sent the caller info by udp over the lan and popped the customer info on the ops pc screen from our db. Worked very well. We want to go over to VOIP so how could I achieve call popping on VOIP and SIP.
Using C# you can manage SIP protocol with https://sipsorcery.codeplex.com/
We use this library for our SIP voip phone.