In order to secure these sensitive communications the Americans designed and built the Sigsaly device. The NSA website says about Sigsaly:‘The SIGSALY system was inaugurated on 15 July 1943 in a conference between London and the Pentagon (the original plan had called for one of the terminals to be installed in the White House, but Roosevelt, aware of Churchill's penchant for calling at all hours of the night, had decided to have the Washington terminal moved to the Pentagon with extensions to the White House and the Navy Department building.) In London, the bulk of the SIGSALY equipment was stored in the basement of Selfridges Department Store, with an extension to Churchill's war room, approximately a mile away……….. With the coming of SIGSALY, the shortcomings of the less than effective A-3 were now a thing of the past’.
This doesn’t appear to be the whole truth. While it is true that the system was installed in July 1943 it didn’t work properly till late 1943 and it only become fully operational in April 1944. Even after it was installed officials continued to use the A-3 for most of their communications since the only Sigsaly link was available at the Cabinet War Rooms and only a small number of officials had authorization to use it.This information comes from the book ‘The woman who censored Churchill’, p112-3. I’ve added this information in Intercepted conversations - Bell Labs A-3 Speech scrambler and German codebreakers and German intelligence on operation Overlord.