http://www.rfc-editor.org/rfc/rfc7255.txt
RFC 7255: Using the International Mobile station Equipment Identity (IMEI) Uniform Resource Name (URN) as an Instance ID, A. Allen2014-05-01T23:00:00-00:00This specification defines how the Uniform Resource Name (URN) reserved for the Global System for Mobile Communications Association (GSMA) identities and its sub-namespace for the International Mobile station Equipment Identity (IMEI) can be used as an instance-id. Its purpose is to fulfill the requirements for defining how a specific URN needs to be constructed and used in the '+sip.instance' Contact header field parameter for outbound behavior.