This Standard defines the Stage 2 (architecture) and Stage 3 (protocol) specifications for the interconnection of an IP Multimedia Subsystem (IMS)-based Next Generation 9-1-1 (NG9-1-1) Emergency Services Network with legacy and other NG9-1-1 Emergency Services Networks for initial emergency call origination and call transfers (bridging). This Standard is incremental to ATIS-0500032, ATIS Standard for Implementation of an IMS-based NG9-1-1 Service Architecture, in that it focuses on the interactions between the IMS-based NG9-1-1 Emergency Services Networks and other emergency services networks. ATIS-0500032 includes the architecture, functional elements, call flows, protocols and interfaces which were derived from the Stage 1 requirements in ATIS-0500023, Applying Common IMS to NG9-1-1 Networks. This Standard expands upon those principles to specify interactions between emergency services networks.