I'm Playing with My Phone in Another World

One morning, the male protagonist was on his way to school with his girlfriend. A septic truck came rushing towards him. In order to save his girlfriend, the male protagonist successfully traveled through time. He came to a place with magic, armor, swordsmanship, dragons, and monsters. How should the male protagonist open his harem? How to return to the original world? Explode this world! This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.