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.
Julio patiently waited by the phone is a main clause. Hoping to hear is a present participle phrase. A present participle phrase will begin with an ing word like hoping. When a participle phrase follows a main clause, you often use a comma to connect the two.