Go to Settings>FaceTime>iPhone Cellular Calls (or something along the lines of that)>Off.
And by the way, Handoff is different from the feature you are describing. You are describing Continuity, which allows your iPad and Mac to pick up and make normal calls, and receive and send SMS. Pretty much makes your life easier so you don't have to always reach for a certain device to do something, just do it with whatever you have immediate access to. Anyway, Handoff allows you to pick up what you're doing on another device. So, you have Safari open on x webpage on your iPhone, turn on your iPad, and in the bottom left of the lock screen a Safari symbol is there, able to be dragged up like the camera shortcut. And boom, the page is now open on your iPad, you "Handed off" the task. Handoff and Continuity are totally different, just had to point that out to possibly clear up some confusion.