In my experience, it's just about impossible in the U.S. (Mostly I've been told it's not permitted for security reasons, or that the FAA frowns on this.) I've never been successful in having an airline convey a message to a passenger.

Assume you can only contact someone who wants to be found. He/she should carry a cell phone/pager in airports; in the air, the passenger must "activate" an onboard phone on each flight segment in order to receive incoming calls.