Halloween Approximately - S2-E2
Other mistake: In the scene where boys are loading eggs into their slingshots, one of the eggs misses and falls on the roof, but doesn't break. A real egg would have broken.
Other mistake: At the beginning of the episode when they are driving in the car, the door is opened to make room for the camera outside but the opening is visible in shot. (01:49:00)
Other mistake: The book Reese is sawing through in the background, while Malcom is talking into the camera, is already cut in half and the saw is not sawing anything. Then they cut to Reese about 7 seconds later as he actually saws the book in half. (00:02:13)
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.