I had that too because I didn't realized that incoming calls are not unlimited. So, when someone calls you they also take your minutes so it might be that you used all the data even though you didn't call someone but someone called you.
The best one in my opinion is the Autopay reward, you just give them your credit/depit card number and you get $2 off, every month. It's also super handy because you don't need to worry about paying because you can't forget.