Warning for iPhone users! Go to your settings > cellular and TURN OFF Wifi Assist. This feature is defaulted to ON and will switch to data usage if your wifi signal is weak. We ended up with 7 extra gigs used last month and 16 extra this month. After trying to figure out what the problem was, we finally contact AT&T today. Neither customer service nor tech support was helpful, and they won't give us a credit because they said since we knew about the excessive usage we should have brought it up earlier.
The credit department employee brought up the Wifi Assist. This is a problem they know about, and that they know is not our fault. Apple is even being sued over it. They still won't credit us, and won't let us spread out the payments over more than a month.
Learn form our mistake. If you have iOS9 and don't have unlimited data, turn off Wifi Assist!!!