1. A method of operating an internet protocol (IP) multimedia subsystem, the method comprising: in a call session control function (CSCF), receiving a session initiation protocol (SIP) registration message transferred by a user communication device and responsively transferring a registration request to a home subscriber server (HSS); in the HSS, processing the registration request to obtain initial filter criteria (IFC) for the user communication device, wherein the IFC comprises processing instructions indicated using a markup language that direct the CSCF to perform an operation to dynamically obtain and load data into the IFC; in the CSCF, receiving a SIP invite message transferred by the user communication device; in the CSCF, processing text from the SIP invite message against a set of keywords to determine a dynamic parameter based on a keyword match; in the CSCF, processing one of the processing instructions in the IFC to obtain and load the dynamic parameter into the IFC; in the CSCF, processing the IFC with the dynamic parameter to determine a communication session parameter; and in the CSCF, transferring a SIP message with the communication session parameter responsive to the SIP invite message.