Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
iChat allows you to talk (have an audio conference) with another iChat (or, I believe, MSN/AOL/etc.) user, but you can't physically call another person's phone.

Skype is built for the sort of thing I think you're looking for, and is free for Skype-to-Skype interactions. There is also the option to pay for the ability to call real phone lines.
 
iChat's audio/video is pretty good but it only works with AIM/iChat users. It doesn't work with the MSN/Yahoo Jabber clients.

I introduced my mother to IM a few weeks ago and sent her a video chat request earlier this week (one way). She was amazed by the quality and sound (not that it took much to impress in this instance) but about 20 minutes in, she started worrying about how much it must be costing me (it's UK long-distance) until I told her it was all going across the internet in her usual broadband package.

Family iSight number 2 now under serious consideration.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.