The present document describes the architecture and functionality required to support enterprise and corporate services as IMS applications hosted in the NGN operator's network on behalf of an enterprise (Hosted Enterprise Services). The present document also specifies the protocol requirements for the UE to attach to the NGN (in particular the IMS) and also any protocol requirements related to application servers provided in support of hosted enterprise services. The structure of the present document reflects the historical structure of previous versions of it. All references to ETSI TS 124 524 [25] are intended to be to the entire contents of the referenced document@ which now forms the contents of the present document@ and not to any part which might be implied by any headings within the present document.