I have just downloaded this C# Command Line Caller sample program of Ozeki VoIP SIP SDK: http://www.voip-sip-sdk.com/p_163-c-command-line-caller-voip.html
It is very easy to use and meets my requirements, but I would really appreciate some extra info from more experienced developers. Has anyone ever built a C# Command Line Caller?

What do you mean by 'Command Line Caller'? Are you looking to write an application that accepts command line arguments, an application that launches applications with command line arguments, or something that places VOIP phone calls from the command line? All three are pretty simple to accomplish (with a good SDK for the 3rd one, that is) but have very different implementations.

Third one :-) Thanx for your answer. Have you got experiences with good SDKs? I think this Ozeki VoIP SIP SDK is pretty good.

Nope, none whatsoever lol. This looks like a good place to start, though.