http://www.rfc-editor.org/rfc/rfc4694.txt
RFC 4694: Number Portability Parameters for the "tel" URI, J. Yu2006-10-01T23:00:00-00:00This document defines five parameters in the "tel" Uniform Resource Identifier (URI) to carry the number portability (NP)-related information. Those parameters can be passed to the next-hop network node after an NP database dip has been performed. [STANDARDS-TRACK]