1. have a friend at the phone company look it up for you in their database.
2. do a Google or Yahoo search.
3. use a reverse cell phone owner directory.

The first of these options may be difficult, as most of us don't have friends in these positions or, even if we do, don't want to put those friends in an awkward position with their employer. The second of these choices only reveals information in the public domain and is mostly ineffective in finding private and unlisted cell phone numbers (likely just the type that you're confronting). The third option, however, gives you an excellent chance of getting just what you're looking for.