The expression "have one's number" typically means having knowledge or understanding of someone, their habits, or their behavior patterns. While there is no direct antonym for this phrase, some alternatives to express lack of familiarity with someone include "don't know someone from Adam," "haven't a clue," "can't put a name to a face," or "blank slate." These expressions suggest a lack of knowledge or understanding of a person, as opposed to having an established recognition of their behavior or personality based on previous experiences. In essence, they offer a negative counterpart to the notion of having someone's number.