Can you make calls using iPad?

No, unless you have a third-party application. You must also have an active internet connection. That means on the WiFi model, you'll need to be connected to an access point, and on the WiFi+3G model, you'll need to have a cellular connection or WiFi access. The iPad itself (in it's 3G flavour) has no calling ability. However with the addition of VoiP providing applications, you may make calls with your iPad. Check the App Store for options.