When I was little, I saw on Cartoon Network a movie with a kid that talked on the phone with a friend of his. Then he turned into a cartoon, got a red car and started a row of adventures. The first adventure was falling asleep in the car and missing a turn on the beautiful road, so that he entered a swamp. At one surtain point, he learned about infinity. When the story from the cartoon ended, he woke up (the image was on film now) and his friend was still on the phone, telling him that he was only gone for a minute. Do you have any idea how the movie is called? Thanks!
OK, I think I found it: The Phantom Tollbooth from 1970. A kid talks on the phone then turns into a cartoon and goes on adventures in a red car (and gets stuck in a swamp). In the end, it's live action again, and his friend tells him on the phone that he was only gone for 5 minutes. It's based on the book by Norton Juster. The entire movie is available on Youtube here.