There's quite a few programs to let you make and receive calls on the iPod touch. All have different costs.
Skype will cost you $2.99 a month for unlimited calling to US numbers. Extra money if you want a phone number and want to be able to receive calls as well as make them.
TextFree is free for texting and real cheap for calls. Allows you to text AND call in and out. But so far for me, calls in have been very unreliable.
Whistle seems to work well for people as well. Calls in and out. He downside to Whistle is you can't pick your own area code. I live in new york and have a Texas number.