I have a 5s and my wife has a 5. According to the AT&T app, we've used 1.62MB of data today, but we're home on wifi. The AT&T rep claims that when an iPhone's screen goes dark and is unplugged, it starts using cell data as wifi disconnects. Is this true? My phone is powered off at night, so it's only been turned on for 2 hours. I have the 2GB plan, so that's the only reason I'm concerned.