A digital telecommunications system, a method of managing a communications network in such a system and a program product for managing audio transmission in a digital communications system. A request to install a target operating environment on a computer system is received at an application. 5% ZrO2, 0-3% TiO2, 0-3% CeO2, 0-2% Sb2O3, 0-2% P2O5, 0.