I know of two apps that can act like phones: Whistle and Talkatone. When you sign up with Whistle, you get your own phone number, while Talkatone uses your Google Talk number (that you make if you don't have one). They work like a regular phone, in that you can dial other cell phones or landlines. You are not restricted to the part where the person you are calling has to also have the same app or program (such as Skype).
They work well for "in a pinch" calls - but, IMNSHO, I wouldn't rely on them for your only phone usage outside your home. YMMV.
Have a look in the App Store, I am sure there are more apps that will allow you to pretend your iPad is a phone. These are the only two I knew of off the top of my head.
Good luck in your search.
Marilyn