Telezonia (1974)

Four children want to invite their friends to a picnic, but they don't know how to use the telephone. Suddenly, the room goes dark and the phone becomes large enough for them to climb into. They walk through a tunnel and meet a man named Telly, who takes them into the world of Telezonia, where they are shown various kinds of telephones. They meet several costumed characters, such as Question Mark, who teaches them how to answer the phone; Q and Z, who show them how to use the phone book; and Exclamation Point, who teaches them how to place a call. By the time they leave Telezonia, they are full-fledged telephone users.

Unfortunately the movie Telezonia is not yet available on HBO Max.