A method for modifying the contents of session initiation protocol (SIP) messages is presented. The method includes receiving a SIP message. The SIP message may include a set of message header fields. The method includes receiving an application policy. The application policy may specify how to modify the SIP message based on a characteristic of the SIP message. Alternatively, the application policy may be retrieved from a database such as one provided by a home subscriber server (HSS) or an application server. The method includes using the application policy to modify the SIP message resulting in a modified message, and sending the modified message.