Take another look at the sentence:
After a successful interview at a bookstore, Julio patiently waited by the phone, hoping to hear that he had gotten a summer job, every call, though, was a twelve-year-old boy calling for Rosita, Julio's younger sister.
Every call was a twelve-year-old boy is a main clause. Though, in this sentence, is an interrupter because it is breaking the flow of the sentence. An interrupter needs a comma both in front and behind it.