n over a next-generation net- work. Although ENUM will help facilitate VoIP calls, it is important to understand that VoIP phone calls do not require ENUM, and ENUM implementation is not mandatory, at least not at this point. VoIP calls can be made wholly without ENUM by using their defined Internet addressing scheme (e.g., sip:user@host.com). The Future of ENUM The Internet Architecture Board (IAB; www.iab.org) and ITU Study Group 2 are discussing a collaboration on the operational administration and delegation issues related to deployment of ENUM-based services. As you can imagine, this requires extensive consultation with administrators of resources derived from the interna- tional E.164 numbering plan, including national and international numbering plan administrators. Under current practice, service providers generally develop their own address- mapping solutions and negotiate their own traffic exchange arrangements among themselves. Experts predict a gradual shift toward public ENUM as the number of IP voice endpoints grows. The following ENUM structures are possible: Private ENUM-Service and equipment providers are finding ways to make money on private ENUM, or ENUM-like, proprietary mapping and signal- ing. ENUM creates and enables the opportunity for a third party to enter and build the technology and then make money by charging fees to all the different providers who have numbers stored in the system. In this sce- nario, the third party would make fractions of a penny per number, per month. A grander vision would include transport service between VoIP pro- viders, combining call routing and transmission. The service providers would have to have a sufficient number of customers and amount of traffic to warrant such services. User ENUM-User ENUM gives end users on the Internet and end users on the PSTN a possibility to find services of other end users on the public Internet. In this case, ENUM service providers provide a form of electronic business card or basic buddy list. The phone number becomes a univ