Continuity lets you answer a phone call on any Apple device
New in iOS 8 and OS X Yosemite is a group of features Apple refers to as Continuity. One feature allows you to make and receive phone calls on every Mac and iPad you own. It’s surprisingly handy, especially if your iPhone is charging across the room or buried in a bag.
However, if you want to opt a device out of sharing phone calls, this feature is governed by FaceTime. On an iOS device, go to Settings > FaceTime and disable iPhone Cellular Calls. On a Mac, open the FaceTime app, go to FaceTime > Preferences, and disable the feature there.