VOIP/SIP desktop app integration

Hi all,
bit of a newbie question here…

I want to find out how I can integrate SIP/VOIP into applications; a simple example would be an application running on my PC which, when a certain number is received, launches a the web browser at a specific URL.

What are the options for approaching this problem?

I guess my three approaches are:
1) use some sort of SIP SDK to build an application to do this (what are peoples experiences of SDKs?)
2) maybe some softphone like X-Lite have some sort of extensible functionality I can harness?
3) maybe there are general SIP macro apps which sit and listen on your pc and then perform an action of your choice on a certain event?

any thoughts appreciated

regards

Joel Hughes