Trying to find out what movie this is?

This was a movie that I only saw the last half hour of on TV back when I was a kid (circa 1985 or so). The main part of the movie was animated, and there was this boy that I think was blond. I remember seeing him traveling around in this underground looking area. Then everything changed and it was a live-action film, and there were two friends that were on the phone. The one boy (who had been the animated one) came back and said, "Wow, sorry for being gone for so long!" and the other one said he had only been gone for 5 minutes. Then there was this red and white striped box in the room, and it flew out the window or disappeared, and then the kid on the other end of the phone yelled, "Hey! This red and white box just flew into my room!" and it looked like you were supposed to ume that he had a similar adventure as the first kid did. That's about all I remember about it. Does this ring a bell for anyone? What movie is this?