http://www.rfc-editor.org/rfc/rfc3939.txt
RFC 3939: Calling Line Identification for Voice Mail Messages, G. Parsons, J. Maruszak2004-12-01T23:00:00-00:00This document describes a method for identifying the originating calling party in the headers of a stored voice mail message. Two new header fields are defined for this purpose: Caller_ID and Called_Name. Caller_id is used to store sufficient information for the recipient to callback, or reply to, the sender of the message. Caller-name provides the name of the person sending the message. [STANDARDS-TRACK]