Ios 8.3 caller screen keeps changing?

Hey so i have the iphone 6 and when someone calls the screen sometimes has two buttons (accept and ignore) and sometimes there's a slide to unlock. Why does the screen keep changing and how can i stop it from changing?

If your phone is locked it shows slide to answer, you can ignore it ( the call ), or slide it so your pants don't answer the phone. When you are using your phone, you can just tap to answer, or if you want to go back to what you were doing, decline. When it's locked you can just press any other button or ignore it no problem.

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