Question: What format should a SIP URI for an IMEI based identity take, according to the specifications?
Option 1: sip:user@ipaddress
Option 2: sip:user@domain where the user part contains the IMEI encoded according to ABNF of imeival
Option 3: sip:imei@operator.com
Option 4: sip:user@ims.mnc015.mcc234.3gppnetwork.org
Explanation: According to the specifications, a SIP URI for an IMEI based identity should take the form sip:user@domain where the user part contains the IMEI encoded according to ABNF of imeival as defined in IETF RFC 7254. This ensures that the IMEI is properly formatted and can be used in cases of unauthenticated emergency sessions.