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Roaming for voice, the ability for a user to make and receive calls when visiting another country, is taken for granted by GSM users today. Currently GSM mobile operators are introducing packet-based data networks (also known as General Packet Radio Service) that bring a huge variety of services. However, this new network also introduces new complexity in the support of the packet-based roaming scenario. Due to the number of services, operators, and users envisaged, it will be impossible for operators to test all services for roaming in all partner networks for GPRS; instead, the capabilities of the network have to be tested. Furthermore, mobile data service developers must be aware that the network capabilities of a visited (GPRS) network might differ from the home network; thus, consideration is required to ensure service continuity. This is especially true as mobile operators start offering 3G networks and support roaming agreements from 3G to 2G networks. In the continuing development of the mobile network, 3GPP (the international standardization body developing the UMTS standards) has defined the IP multimedia subsystem (IMS) as an evolutionary path for basic 3G networks (developing on the already defined Release 99 and Release 4 capabilities). The IMS provides an overlay architecture on top of the basic 3G architecture and implies roaming outside the basic 3G architecture. The resulting 3G-IMS network creates a two-layered roaming architecture, potentially resulting in issues related to optimal routing, QoS, and support of local services. The challenge for the industry cannot be pointed out as one difficult technical issue that needs to be solved to ensure 3G roaming. Instead there many smaller issues with many alternative solutions that need to be solved. Only when operators have a clear view of these issues and their roaming requirements can all issues be solved and roaming agreement negotiations run smoothly. As it looks today, it will be time consuming to establish and conclude the roaming agreements for 3G; thus, delays in the roaming service are expected. When IMS arrives, new addendums will be needed to the roaming agreements, and the same risk for delays exists if the demand issues are not properly addressed before the ne- gotiations.