|"To dial numbers and do any kind of interactive voice response you need a telephone voice card"|
Not necessarily (on the first part). The dialer application that comes with XP is not designed to be used for actually speaking with your party - it is only designed to dial the number. The idea is you have your PC connected to your phone line and you have a phone next to your computer. Instead of having to look up someone's number and dial it into your phone, you simple select them from within the dialer application and click a button. You can then pick up the phone to talk to them.
It's not the most elegant solution, but the fact that most modems have a passthrough connection it does seem reasonable. I would use it except I haven't have a landline phone in my house for about 5 years now. Although, I was experimenting with this the other day and using bluetooth I can use the dialer app to place a call on my cell phone. But, the interface on my phone is much easier to use anyway.