Weill... they have his flight records, so they'll be able to tell if he's out of town on a flight booked through UA. And area codes can be looked up to tell what state the phone originally was (reported to be) in when the phone company issued the number.

It's safest to assume that a phone is at it's correct time zone. People can't get nearly as annoyed if they receive a call at odd/unfriendly hours when they're travelling.

Additionally, there's always email.