ture that enables them to comply with this requirement. Chapter 5: Components of SIP Trunks 69 The SP that currently has TDM infrastructure and can provide these mandated supplemen- tary services via the TDM infrastructure often reuses this infrastructure to provide the same services to their SIP trunking service. In this method, all calls, regardless of whether they can stay within the SP IP cloud completely, are sent to the PSTN for billing and Legal Intercept. This enables the SP to reuse the systems in place to legally intercept and report to authorities any information about a call; however, it hinders the capability of the SP from innovating on services because all calls need to terminate on the TDM network. A second method that can be used to accomplish the goal of fulfilling the legally required supplementary services is to acquire a purpose build IP-based Legal Intercept Server. This supplementary feature service would provide the ability to terminate the request for a Legal Intercept or "tap" from the authorities, configure the call agent with any special requirement to enable the tap to start, and convey the captured information to the authorities. This type of dedicated supplementary service server is used only in a small percentage of calls but has an important purpose that is mandated by law. A third method that has failed in many countries is an attempt to get a waiver for SIP trunks to identify them as a distinct feature that should not be required to comply with the existing laws. This argument has not been successful in most cases and any SP who will be a long-term provider of a SIP Trunk service needs to have a method to implement required supplementary services. Table 5-3 outlines the different method discussed for implementing legally required supplementary services. The second legally required service that is often mandated is emergency calling. In the United States and most western countries, there is a single, countrywide number to call to summon help. The routing problem that occurs with 