The first steps are the most difficult in general but I could avert my all doubts easily. Anybody can build a basic softphone but developing one with extra features requires a good SDK. I can find everything in one library, I can combine each projects that I needed.
The only thing you have to do to follow the next 10 steps:- Creating a Windows Forms application in Visual Studio
- Use the methods of SDK
- The appearance of the Softphone
- Initializing the main parts
- How to register your softphone to the PBX
- Define the SDK events
- Reaction when a button is pressed
- Sending DTMF signals during a call
- How to properly with signing up and down
- Check the created application
If you wish to know the detailed steps then please click on "Softphone development".I hope this information was useful for you.