http://www.rfc-editor.org/rfc/rfc4458.txt
RFC 4458: Session Initiation Protocol (SIP) URIs for Applications such as Voicemail and Interactive Voice Response (IVR), C. Jennings, F. Audet, J. Elwell2006-04-01T23:00:00-00:00The Session Initiation Protocol (SIP) is often used to initiate connections to applications such as voicemail or interactive voice recognition systems. This specification describes a convention for forming SIP service URIs that request particular services based on redirecting targets from such applications. This memo provides information for the Internet community.