The Handoff and Continuity features in iOS 8 allows you to share unfinished work, phone calls and messages on all your Apple devices. This means your iPad will accept phone calls and messages when it is connected to the same Wi-Fi network as your iPhone. This is a very useful feature but can be annoying if you're trying to work, read or play games on your iPad without being disturbed.
One way to disable this feature temporarily is to use the Do Not Disturb feature. You can turn on this feature in the Do Not Disturb section in your Settings. Just tap the Manual slider to turn it on or tap the Scheduled slider to schedule a time for it to turn on and off.
You can also disable the Continuity feature by navigating to Settings > FaceTime and tapping the "iPhone Cellular Calls" slider so it is OFF/WHITE.