I recently downloaded the Whistle Phone App onto my iPad, and I highly recommend this app to anyone with an iOS device. It lets your iOS device become a phone wherever there is Wi-Fi, for free! And you even receive a phone number for other people to call you iPad, using push notifications! Although it seems like it would be awkward to use such a big device as a phone, I avoid this problem by holding it in front of myself, more like a speakerphone.
The main screen of the app is very much like the iPhone’s number pad, but with some extra buttons on the side, such as Line 1/Line 2 buttons. At the top of the screen, it has buttons for Recents, Contacts (Phone numbers from your Address Book), and Bonjour, for calling other Whistle devices on the network.
Whistle Phone is available for your iOS device on the App Store, or even for your Mac or PC at whistlephone.com.