Looking up a phone number on Google sometimes works, especially for people with land lines or businesses, but nobody uses a land line anymore. The problem is that there's no official directory for mobile phone numbers.

There are also sites available that don't require any information from you. In my experience they're pretty accurate and have a broad database. I've yet to do a search that hasn't turned up results. My favorites are www.cellreversedirectory.com, www.namefromcell.com, and www.reverse-white-pages.com. If you really need to find somebody or find out who's calling you these may be your best bet. Best of luck