Kicked Out - S4-E12
Continuity mistake: Malcolm's seat belt changes from on to off the first scene with him and his mom in the car. When it's a shot from the front it's on, but when it's a shot from the side (from the driver's/mom side) you can see that the part of the seat belt that's going across is now behind Malcolm, instead of on him like it was a second ago. This changes back and forth.
Answer: Francis is only allowed to talk on the phone at school for three minutes a day. I think the code gives him free access to talk to his girlfriend.