Take a look at the sentence again:
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.
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.
Go back to the sentence to try again.