if you are in the US then google voice is free.
1) set-up a Google Voice account - totally free. Get yourself a new number on it even.
2) Download the talkatone app - also totally free. It has adds which you can remove for $.99.
3) set-up talktaone to make calls from googletalk and login with your google account. Turn on notifications and bam, you will receive calls and can make calls through the ipad for free - inside the US. to cell phones, landlines, etc.
I do this every day, I don't even have a cellphone besides my iPad. I don't have a home phone. I have my office phone and my ipad2.