IMS, or IP multimedia subsystem, is at the heart of equipment suppliers' and operators' plans for converging fixed and mobile networks. Proponents suggest it is set to become the linchpin of all-IP multimedia, and thus the great revenue-generating hope for operators in the second half of this decade. A standards-based technology, IMS has been designed and developed to enable service providers to offer services based on the session initiation protocol (SIP), such as voice over IP, push-to-talk over cellular, and video messaging. A key factor is that these services could now be provided across multiple networks, regardless of their fixed or mobile nature. The article discusses how the system works the plans of different service providers.