As you know iPad don't have inbuilt phone functionality but you can still make call to mobile phone or land line phone using your iPad. Thanks to VoIP technology. Since your iPad has internet connection through WiFi or 3 G network. You can turn your iPad into a phone using VoIP apps. Most of the household has WiFi, that makes it easier to use this to save money.
If you are mostly making calls from home or place where WiFi is available, you don't need to subscribe to 3G data plan. If you travel a lots or make lots of phone calls on the road, you may consider to get a data plan from your cellular provider.
Now the questions is where to find the VoIP app to make phone calls from iPad?
In your iPad, locate the icon App store. You can also search for "App Store". Go to App store search for "VoIP call", you will find lots of free VoIP apps to make phone calls on iPad. You can get one with good customer review and good rating.
Let me walk you through one VoIP app, I have been using to make phone calls on my iPad.
Go to App Store, search for "dial91 VoIP". select "dial91 VoIP", download the app by click on "FREE" button. Now sign into your app iTunes account and install. Once it installed, select "Dial91 VoIP" icon to open the app, click on Accounts tab to enter your dial91 PIN. Now click on "Dialer" tab to enter phone number to call, select Call"
Happy Calling from your iPad!!!