My iPhone started doing this after the last update. The phone app shows up when I'm clearing off open apps. It's blank and cannot be swiped off. I can only get rid of it by a hard reset. Anyone else seen this? More buggy software from Apple.
It does throw up the usual age old question of why you are clearing open apps though when you dont need to? It makes no difference at all and arguably wastes more battery according to some folk.