Complex sentence: Most calls come not from campus pub patrons as one would expect as they are generally in groups, but from lone students doing school work at the library or in a residence late into the night hours.
Split into simpler sentences: Most calls come not from campus pub patrons as one would expect as people who frequent the campus pub are generally in groups. Most of the calls come from lone students doing school work at the library or in a residence late into the night hours.