Saw it too late, perhaps
Because this movie was made in '74 and I wasn't made until '82 and didn't see it until last week, its characteristic, inspired themes became passe and considerably old hat in just the ensuing years, i.e. the end of the 70s, much less into the great-great-great-great-(etc.)-grandchildren: I Know What You Did Last Summer, Wrong Turn, et al. The core ideas (the killer being in the house, gatherings of nubile, vulnerable girls, obvoius distractors (red herrings like Keir's character) and such became textbook lesson, sure-fire blueprints for a killer movie a long time before I saw it, and consequently I just didn't get jived by it. The discordant voices on the phone were unnatural and haunting, but the killer being in the house was suggested aready by the fact that he used the same phones the girls had. I hated how we didn't see this "Billy" nor understand why in heaven's (or hell's) name he tore into his deadly rage. Had I seen this in '74, however, it would look and sound different today, but I missed it by...33 years.