We use 3CX as a corporate PBX and I need to create a C# SIP telephone application with some extra features (autodialing, conferencing, call routing and some additional call center functionalities) that will be used to make huge amount of simultaneous calls through our PBX.
I didn't find any proper C# API for 3CX, so I think I need to use a third-party C# library suited for this purpose to implement the VoIP backgroud of the application. I am newbie in this field and I need some suggestion on which library is the most suitable in my case.

I found many related tutorials on the web that uses Ozeki VoIP SIP SDK. According to this page it's 3CX-compliant: http://voip-sip-sdk.com/p_124-3cx-pbx-setup-voip.html. It looks good, but this is a commercial product... and I'm afraid, its trial period is not enough. So, I'd rather use an open source one - if available at all.

What do you think about this? Do you have any experiences with Ozeki VoIP SDK in long term in corporate use? Can you recommend me a free alternative to this library?