The communication device E.T built to phone home? Totally legit, as described in an article in the April 1983 edition of “73 Magazine” (an amateur radio magazine):
If it were not for an inventive ham, E. T. might still be trying. In this exclusive article, the designer of the little guy’s communicator unveils its inner workings.
Always great to hear/see if a technology used in a movie is factually correct, unlike things like CSI’s zoom and enhance for example.