company (that is, Company A) and a phone number for the company at which they are employees (that is, Company B). One idea that is possible with this arrangement is to have an IP phone associated with the IP-PBX that belongs to both Company A and Company B. This idea of having a single extension that is homed to the IP-PBX at Company B to a user in Company A is possible; however, it does pose challenges. These include the inability for most IP Phones to respond to multiple call agents, the lack of a widely used method to authenticate an IP Phone against an IP-PBX, and the use of NATs or firewalls that would prevent incoming signals that would be required for registrations and to allow the phone to ring. However, this method would have distinct benefits over other methods of collaboration because it can be monitored and controlled by the IT department using the same methods in place for intercompany communications. Because the capability to have an IP endpoint controlled by an IP-PBX at multiple com- panies is problematic, one option that companies have examined and are beginning to adopt is the use of SIP trunking between enterprise PBXs. This service enables companies to take advantage of the current IP-PBX features, such as conferencing, billing, and Chapter 2: Trends in IP Telephony 23 recording of calls, but if federated could provide connectivity beyond the basic walls of the enterprise. As such, there is a trend occurring in which companies will adopt a SIP trunking solution between each other to take advantage of many of the benefits of SIP trunking without giving up the independence of their IP-PBX. This intra-enterprise SIP trunking carries with it many complications in addition to benefits. The most basic complication is the issue of IP address reachability. The basic premise of a trunking solution is that a shared IP address exists between the two entities. However, many UC communications solutions use private IP addressing schemes. As such, enabling these to interconnect requires the use of a shared I