The cell phone number is more than just a bunch of digits. It is increasingly used as a link to private information maintained by all sorts of companies, including money lenders and social networks. It can be used to monitor and predict what you buy, look for online or even watch on television.
It has become "kind of a key into the room of your life and information about you," said Edward M. Stroz, a former high-tech crime agent for the F.B.I. who is co-president of Stroz Friedberg, a private investigator.
Yet the cell phone number is not a legally regulated piece of information like a Social Security number, which companies are required to keep secret. And we are told to hide and protect our Social Security numbers while most of us don't hesitate when asked to write a cellphone number on a form or share it with someone we barely know.
That is a growing issue for young people, since two sets of digits may well be with them for life: their Social Security number and their cell phone number.
Among people ages 25 to 29, the share of homes that have only wireless phone service stands at 73%.
Investigators find that a cell phone number is often even more useful than a Social Security number because it is tied to so many databases and is connected to a device you almost always have with you, said Austin Berglas, a former F.B.I. agent who is senior managing director of K2 Intelligence, a private investigator.
"The point is the cellphone number can be a gateway to all sorts of other information," said Robert Schoshinski, the assistant director for privacy and identity protection at the Federal Trade Commission. "People should think about it."
But if a cell phone number and the private computer behind it open the door to new risks, technology, as is so often the case, can also be employed to combat those risks.
"What you can do with the cell phone number and mobile technology represents a pretty substantial advantage in the ongoing war against fraud and identity theft," said Rajeev Date, a venture investor and former banker, who was previously deputy director of the Consumer Financial Protection Bureau.
But a cell-phone-only life presents problems for many independent professionals and workers at start-ups and small businesses, who make business calls on their personal cellphones. But now, professionals and other mobile business people can turn to a new app Sideline to add a second number to their cellphones so their personal number remains personal.
The service is free for individuals and $10 a month a number for groups of workers in a business, who get extra features like a company directory and voice mail transcription. One of Sideline’s ad mottos is: “Keep your personal number private. Add a second number to your smartphone.”