There is a DVD so I suspect they did make a movie out of it, but it started life as a book. And as we all know, don't judge a book my its movie.
from an interview with Norman Juster, author of TPT:
RES: Wouldn't The Phantom Tollbooth also lend itself perfectly to animation?
JUSTER: It was done as a full-length animated feature film by MGM. With live scenes. Live beginning and end. And when you go through the tollbooth is when it turned into animation. It was a film I never liked. I don't think they did a good job on it. It's been around for a long time. It was well reviewed, which also made me angry. And for some reason, I don't know why, they never put it into general release. But it played a lot and it still does, on television.
RES: I don't know that I've ever heard, saw, or read about its being on TV.
JUSTER: It's drivel. So don't worry about it. I don't own the rights anymore, but the conglomerate that owns it, MGM, Turner, Time Warner, AOL, they're now very seriously considering remaking the movie as an all-live action film. That's the way I'd prefer it. Whether it happens or not I don't know. I've had enough dealings with the crazy movie world to know that a vast majority of things never happen as they say. And if they do happen, they happen at a glacially slow pace. What I try to do is put it out of my mind. If it does go ahead, I hope I will have some function in it because I'd like to do it in a way that I think it should be done.