Enhanced registration messages in internet protocol multimedia subsystems

An enhanced Session Initiation Protocol ("SIP") registration message having extended header information that is used by an Internet Protocol Multimedia Subsystem ("IMS") core to determine the registration status of a mobile device and the physical location of the mobile device. The extended header information includes hardware and subscriber identifiers, such as an International Mobile Equipment Identity ("IMEI") and International Mobile Subscriber Identity ("IMSI"). The IMS core queries an equipment identity register to validate IMEI/IMSI identifiers in the header to determine whether to deny registration to a mobile device. The IMS core also queries a capability database using an IMEI to determine which location determination techniques are supported by or suitable for the associated mobile device.