"The present document provides a description of user@ functional and some technical aspects of an IP-based text telephony service@ supporting the use of fixed and mobile mainstream devices@ as currently deployed in Scandinavia (Denmark@ Finland@ Norway and Sweden - known as ""Reliable RTT"") and some additional countries (such as Germany and the Netherlands). The present document provides a pointer to the source where the full details of the implementation@ including a private Session Initiation Protocol header and some SIP examples@ are made openly and freely available to any developer@ service provider or other stakeholder interested in the service. The present document carries no operational@ interoperability or other requirements on terminal manufacturers@ software developers or fixed or mobile network operators or service providers or their regulators@ who are not involved in the provision of the service described."