Go into App Store and search for "Google Voice".
You will find a few apps that uses Google Voice, eg "Talkatone", "Google Voice".
You will need to go to http://www.google.com/voice to setup a free account.
You get a free North American telephone number, which you can use to receive and call other North American numbers for free. Each call is limited in duration though (1 or 2 hrs) per call, meaning if you use this to get on a conference bridge, it'll cut out after 1 hr (or 2 hr, I don't remember which), then you'll have to redial in again.
Voice quality is not that great.
When someone calls you on that Google Voice number, your iphone will ring, if you have the app set up in the iphone's notification to receive alerts. You'll need this app to run in the background too.
If the app is not in the background, some app, such as Talkatone, will still ring your iphone (not the regular ring sound, but the app's own ring sound), but it may not connect fast enough.
But even If the app is running in the background, some times the iphone still will not ring. Well, this is the price we pay for using a free service.
So in summary, Google Voice has just average to poor voice quality, and not too dependable incoming call ring.
Pinger is another app that allows you to get a free telephone number, incoming is free, outgoing you pay at the going rate for this type of voice over ip service (about $0.01/min).
Pinger picks up incoming calls more reliably than Google Voice, and has better voice quality. It just costs to call out.
In my opninion, these types of voice over ip calling apps are just for fun. I wouldn't use them to call my boss or customer. Voice quality varies, depending on how far away you are from the wifi router. Google voice is a good alternative for those very very long conference calls if you don't have unlimited minutes, and if you don't have a landline.
Vonage is another voip app that allows free outgoing calls to North America but it cannot receive calls over IP.
I've played with these apps just for fun. In my everyday use I use my regular voice service for work related calls, and my landline IP phone for conference. These voip apps for calling on non-work related chit chat calls.