sentences:  Suddenly the rules had changed and the race was on to develop an electronic telephone exchange that could operate with the current GPO telephones used in the UK, including shared service.Just before World War II, Tommy Flowers MBE, employed at the GPO, had been working on VF (voice frequency) signalling, using valves (vacuum tubes), and this had led him to realise that valves could be very reliable if not switched on and off.This gave him the confidence during the war to build the world's first digital computer, called Colossus, at Bletchley Park.