http://www.rfc-editor.org/rfc/rfc7316.txt
RFC 7316: The Session Initiation Protocol (SIP) P-Private-Network-Indication Private Header (P-Header), J. van Elburg, K. Drage, M. Ohsugi, S. Schubert, K. Arai2014-07-01T23:00:00-00:00This document specifies the SIP P-Private-Network-Indication P-header used by the 3GPP. The P-Private-Network-Indication indicates that the message is part of the message traffic of a private network and identifies that private network. A private network indication allows nodes to treat private network traffic according to a different set of rules than the set applicable to public network traffic.