Telephone directories can only list landline telephone numbers because those phone numbers are a matter of public record. Cell phone numbers, however, are not considered public records, and websites or services offering by-name look-ups for cell phone numbers are either a fraud or obtaining personal information illegally, as stated by Inflection LLC, a company specializing in people searches.
Most people want their cell phone numbers to remain confidential, as noted by Inflection's Phone Detective website. The premise is that people who want someone to have their cell phone number will give it to them, but not doing so demonstrates that they prefer to not be contacted that way. Telemarketing calls made to an individual's cell phone represent a particularly unwanted occurrence, and opposition from consumer groups has so far managed to prevent cell phone numbers from becoming public records.