A communication system comprising: a first client device associated with a user; a second client device associated with the user; a server, the server hosting a service; the first client device connectable with the server to provide a first message to the server to register with the server so that the service is usable by the user via the first client device, the first message having user data related to a user account for use of the service and at least one routable address of the first client device; and the server assigning a service address to the first client device and associating the service address with a single address assigned to the user, the server subsequently sending a registration response to the first client device to identify at least one of the service address and the single address to the first client device such that the registration response provides the first client device with information necessary for communicating to the server for using the service; characterized in that the registration response is a SIP service profile which comprises both the service address and the single address, the service address and the single address being provided in a compound address, the server associating the compound address with the first client device; and communicating said compound address to the first client device such that the first client device provides third party communication session control such as, for example uaCSTA, to manage sessions active on its device as well sessions that are active on other client devices associated to the user, such as the second client device, utilizing the compound address either directly or in connection with communications sent to the service hosted by the server. a first client device associated with a user; a second client device associated with the user; a server, the server hosting a service; the first client device connectable with the server to provide a first message to the server to register with the server so that the service is usable by the user via the first client device, the first message having user data related to a user account for use of the service and at least one routable address of the first client device; and the server assigning a service address to the first client device and associating the service address with a single address assigned to the user, the server subsequently sending a registration response to the first client device to identify at least one of the service address and the single address to the first client device such that the registration response provides the first client device with information necessary for communicating to the server for using the service; characterized in that the registration response is a SIP service profile which comprises both the service address and the single address, the service address and the single address being provided in a compound address, the server associating the compound address with the first client device; and communicating said compound address to the first client device such that the first client device provides third party communication session control such as, for example uaCSTA, to manage sessions active on its device as well sessions that are active on other client devices associated to the user, such as the second client device, utilizing the compound address either directly or in connection with communications sent to the service hosted by the server.