I have found one from SourceForge and one from the Avaya site. The tutorial that Avaya providing is too advanced, so a beginner tutorial would be useful.
Can anyone help?
I would suggest registering on Avaya DevConnect website: http://devconnect.avaya.com. You'll likely find more information there, including product documentation, tutorials and discussion board. Not sure they have tutorial for C# though, but it's worth to check.
@Mamta Dalal: TSAPI is one of the Computer Telephony Integration APIs used to control Avaya phone switches.