The next time someone asks you for your cellphone number, you may want to think twice about giving it.

The cellphone 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 cellphone number is not a legally regulated piece of information like a Social Security number, which companies are required to keep private. 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.