http://www.rfc-editor.org/rfc/rfc7549.txt
RFC 7549: 3GPP SIP URI Inter-Operator Traffic Leg Parameter, C. Holmberg, J. Holm, R. Jesske, M. Dolly2015-05-01T23:00:00-00:00This document defines a new SIP URI parameter, 'iotl' (an abbreviation for Inter-Operator Traffic Leg). The parameter can be used in a SIP URI to indicate that the entity associated with the address, or an entity responsible for the host part of the address, represents the end of a specific traffic leg (or multiple traffic legs).