Get the Vonage for iPhone app -- yes, it works on iPads, too. All calls within or to the US (or any US-based number overseas) are free. The only thing is, obviously, that you cannot receive calls on your iPad, so during the installation process, make sure to specify your cell phone number (or any other number that you have access to at that moment since they will call teh number for a verification code).
The app will ask you for a credit card for the pay-as-you-go option and/or to purchase a monthly plan, but when you get to that point just press the home button and exit the app. You only need to pay for calls if you want to call a non-US number.
Enjoy.