Abstract: An apparatus and method for generating compressed SIP messages from full sized SIP messages and vice versa in order to decrease call set up time in an IP based communication system. During registration of a device, the invention caches the device's static information in the core network in a “Registrar/Location Server.” Subsequently, during call set up, the device transmits its dynamic information to the SIP Agent in a compressed SIP message over an air interface. The SIP Agent retrieves the static information (from the Registrar/Location Server) along with the dynamic information in the compressed SIP message to generate a full sized SIP message. The SIP Agent forwards the full sized SIP message to a SIP Proxy, which is then transmitted to the IP system. Likewise, when a full sized SIP message is received from the IP system, the message is forwarded to the SIP Agent to generate a compressed SIP message for ultimate transmission to the device over the air interface.