Using VoIP programs from my VB application, possible?

Hi, I am just looking for a free alternative to expensive SIP SDKs. I need my application to originate calls using SIP providers.

I am just wondering if it is possible to originate calls from my application using an already developed softphone like X-Lite for example. Lets say, my application requests X-lite to call a particular number. It would also be usefull if my application could somehow know when the call had ended as well as terminating a call if it needs to. Is it theoretically possible?

I mean, you press CTRL-V to paste the number into X-lite and press enter to start a call, that works. So I assume, the application, in theory could also do that automatically for me, no?

I would appreciate any comments.
Thanks.