The present invention relates to a communications system (1) for handling communications sessions, for example multimedia calls or voice calls. The communications system (1) comprises a local terminal (10), an external server (40), a proxy interface agent (PIA) (11) between the terminal (10) and a shared network (20). The communication means includes a NAT function (32) through which the communications session must pass. The communications session is carried over the network (20) over one or more logical channels between the terminal (10) and the external server (40), during which the first NAT function (32) applies network address mappings on the terminal's transport addresses (14). The PIA (11) acts on behalf of the terminal (10) in communications with the external server (40), and establishes a logical channel on an outbound connection to the server that serves as a control channel between the PIA (11) and the server (40). The PIA (11) establishes dynamic outbound connections to the server (40), and in response to a request from the server or in response to a request from the PIA itself (11), makes one or more associations between the terminal's transport address(es) (14) and identifiable logical channel(s) between the PIA (11) and the server. These identifiable logical channel(s) are established on one or more of the dynamic outbound connections from the PIA (11) to the server (40).