You might try
http://www.callwave.com/findoutmore....=nav_broadband -- it seems to be what you want. I've never used anything like this, so I can't speak from experience.
I'd be careful with anyplace. though -- read all the fine print. I think this all has to operate with help from the phone company, so they may require you to change your carrier or something. I can't find any info about how it works, but I assume they provide you with some kind of phone service. I mean, software alone can't do this, because if you have a single phone line, and someone calls when it's in use, the person gets a busy signal and the phone line isn't interrupted or signalled in any way (unless you have call waiting).