When you call someone and the only thing you say before hanging up is something you know will irritate them severely.
An example of cell phone bombing is calling up a friend who is deathly afraid of spiders and yelling "SPIDERS! SPIDERS, EVERYWHERE!!!" and promptly hanging up.
I cell phone bombed my brother, who is very OCD in his dislike for odd numbers, by calling, saying "I see sevens," and hanging up.