Fridge Brilliance - The finale to the series ends without revealing whether or not Chris passed the GED. While it plays homage to The Sopranos, at the same time, it's a clever way of showing that Chris did in fact pass it. At the end of each episode, the punchline would involve Chris either failing at what he had attempted to do or otherwise the butt of the closing joke. In this case, it finally breaks the running gag by ending the show before revealing what happens.
Fridge Logic - Chris's dad had a system to beat the price of collect calls, which the children knew. How come he hung up as soon as he heard the words "You have a collect call from"?
His method was to speak fast enough before the options came up. Perhaps the kids can't speak that fast?
His primary concern is to save money - it's probably just a reflex at that point, and the possibility that it could be one of his kids? Didn't even occur to him.