Here is 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. Every call was a twelve-year-old boy is a second main clause. These two main clauses have only a comma separating them, which is illegal in writing.
The best way to fix this sentence would be to put a period after job and capitalize the E for every.