free windows based voip solution

I am building an applicaiton that needs to answer an incoming phone call, play a .wav file to the caller, capture the caller id, generate an event for a vb app and then send a text message back to the user.

We are currently using a TAPI interface with a regular phone line, but I assume there is a better way uisng VOIP and a soft PBX.  Can you suggest some good solutions for;

1.  Windows based free soft PBX (I assume this is what i need to answer the calls)
2.  I assume I need a IP phone service that gives me a phone number, routes the calls to my app and sends me the caller id.  Can you suggest a good and low cost solution?
3.  Any other good sample applicaitons that would show me the ropes would be helpful.

thanks much.