The IP multimedia Subsystem (IMS) is the evolution of the 3G mobile networks towards new generation networks (NGN) that are only IP based. This architectural framework is seen as a key element for achieving network convergence defining a new horizontal integrated service offering, based on a common signaling protocol (SIP) for all multimedia services such as Voice over IP, Video call, or instant messaging. However the present deployment of IMS is specified according to a specific model, the so called walled-garden. In this model the applications are only provided to the users within the same operator so that the users will not have to look for applications outside the IMS garden. It is a very restrictive access mode for the users because they remain dependent on services offered by the provider and can consequently not choose freely applications they want to subscribe for. The goal of this paper is to include Single Sign-On (SSO) features in the standing IMS architectures to allow the user accessing all the applications, even the external ones transparently, simulating a walled-garden environment. We also introduce the notion of security level that will be affected to the SPs, and implementing it in what we can call “a Multi-level authentication model”.