Phone is the same principal as lights, no? It is prohibited because you are creating something when you use it, correct?
More complex than that, but in a way, yes. The point here is that if something is forbidden to use on Shabbos, it is also forbidden to move it, as once you are handling it, you may forget and end up using it. So with the phone example, you may not use a phone ( for whatever the reason is...), hence you may not pick up or move a phone either, even if it's off, etc.