Why does Caller ID say 'Out of Area'?

The message "Out of Area" was adopted originally by the phone companies because originally long distance companies wanted a portion of the accessed charge to pass along the information.

If there was no agreement to pass the data along, you were out of the area.

If you see a message like "Private Call," the caller opted to block delivery of the Caller ID data. If you see "Out of Area," then either one of the telephone companies along the line "dropped the ball" handing along the Caller ID data, or the call originated from a PBX that was not correctly configured to send Caller ID data, usually because the caller is a telemarketer who prefers the anonymity of not letting you know who's calling.