E.164 phone numbers) is the most important feature. Without this, the phone network would have splintered into a set of incompatible networks that would have required end users to maintain different devices on each network. This, for example, is the case and problem with most two-way radio network and is a major reason that they are not more widely deployed. The only other network with the same level of interconnectivity as the PSTN, and as such with the same level of success, is the Internet. On the Internet, any address entered should result in a path to the endpoint. This same expectation exists in the area of SIP trunking. One way that an SP can ensure that it is connected to the largest number of endpoints at the lowest cost is by exchanging traffic with other SPs via a peering SBC. A peering SBC, unlike an edge SBC, is focused on connections to a smaller number of "peer" SPs. The trust relationship with these SPs is different than the trust relationship between SPs and customers. Because the peered SPs can be both a customer, who pro- vides the SP revenue in exchange for terminating minutes and vendors who charge the SP for terminating minutes, the trust relationship is more complex than traditional cus- tomer/vendor relationships that exist with connections to end customers via the edge SBCs. Because of this complex trust relationship, there needs to be security in place, but it is also much more likely that the two SPs will use a fully routable Internet-based IP address space for the connection, whereas with an edge SBCs, this is much less likely. One aspect of security that is often different than with customers is contractual security. This is security derived from the legal contract that exists between the peered SPs. These contracts can require the SP who is at fault for malicious activity, whether under its control or not, to be liable to the other SP, if any damages result. Though these contracts might not practically provide any additional security, they do exist. SPs that offer SIP trunks generall