Here is the item again:
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. Every call, though, 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 internship and capitalize the E for every.