How can I make calls on Iphone without any cellphone service?

I would like to be able to make and receive free calls on my Iphone using WiFi without getting a cellphone provider. I have an Iphone 3G that's jailbroken and on the latest iOS. I have heard of people doing this but can't seem to find anywhere that has a step by step on how to do it. If anyone could help that would be awesome. Thanks.|||you would have to use skype or another app that allows you to call|||You can use skype or search the youtube for videos that show step by step instructions