k. Examples of these supplementary services include Support for shared or overflow conferencing services in which some conferencing resources are used within the enterprise and some conferencing resources are used in the SP. Support for multicast music on hold support. Support for SNR where a call is placed to ring at two devices at once. Support for encryption of the media between the endpoints; known as Secure Real Time Protocol (Secure RTP). 28 SIP Trunking The advancement of supplementary services over a SIP trunk is most difficult because it requires a close tie between the systems and methods used for these features by the serv- ice provider and the system and methods used by the enterprise. The final example of an advanced SIP trunking service trend is media shuffling. This is a service in which the SP allows the signaling to traverse a separate and distinct path than the media. For many SPs this basic functionality already exists, but the enhancements are that the media shuffling will, in the future, take the most direct path from the endpoint of the customer to the endpoint that needs to be connected at the SP without any cen- tralization of hairpinning of the media. Today's services often have a solution with media shuffling available to the enterprise. The media can come from any location at the enterprise premise to a central location of the SP; however, a true media shuffling solution, in which the media is optimized for both the SP and the enterprise is a trend that has yet to fully develop but will offer improved call quality across all services. Feature Trends for Call Centers Services from SIP Trunk Providers SIP trunking for call centers is a unique challenge for SPs. Call centers were one of the first groups to embrace the utilization of SIP trunking because they used such a high number of TDM trunks. This direct replacement for basic large-scale trunks has been the major, first application provided by many SPs; however, there are unique services required for call centers that are beginning to 