Based on the communicative technologies available, while US and USSR troops were deciding on how the invasion of central Europe would be done, for example, how long would US High Command have to wait before the USSR could confirm? Germany to Italy or Japan, UK to US, etc.
UK to US: Originally there was a secured scrambled telephone line underneath Selfridges in Oxford Street, London that went directly to the White House. This was later relocated to the cabinet war rooms.
Just across from Parliament, underneath a government building are the cabinet war rooms. Constructed in 1938 it was primarily a communications hub. You can read about them here: http://en.wikipedia.org/wiki/Churchill_War_Rooms
There was a special room called the transatlantic telephone room where Churchill would talk to Roosevelt. It was basically the size of a big closet and was kept secret from all but a few who needed to know.
So communication across the Atlantic would be quite quick and presumably ad-hoc?