A user agent (UA) for communicating with a communications network implementing an internet protocol (IP) multimedia subsystem (IMS) is presented. The UA is configured to send and receive session initiation protocol (SIP) messages. The UA includes a processor configured to send a message to the network. The message identifies an application policy. The application policy defines at least one of a SIP message header field to include, a SIP message header field to remove, a SIP message header field to allow, and a SIP message header field to modify. The processor is configured to receive a SIP message from the network. The SIP message includes a set of SIP message header fields. The set of SIP message header fields are modified in accordance with the application policy.