Review the item:
After a successful interview at an engineering firm, Julio waited by the
phone, hoping to hear that he had gotten a summer internship, every call,
though,
was a twelve-year-old boy requesting to speak to Rosita, Julio's
youngest sister.
Every call was a twelve-year-old boy is a main clause. In this sentence, though is an interrupter because it is breaking the flow of that clause. An interrupter needs a comma both in front and behind it.