He will link up with former colleague Tom Alexander, who started as chief executive of Orange UK last month. Day worked under Alexander at Virgin Mobile.
Day was formerly business editor and joint deputy editor of the Sunday Express before moving into comms in 1999 as head of PR at Virgin Mobile.
He became director of corporate affairs for the firm a year later, and left in 2006 year when NTL completed a £962m takeover of Virgin Mobile (PRWeek, 7 July, 2006 ).
Stuart Jackson remains in his role as interim comms director, reporting into Day, which he assumed following the departure of corporate comms director Scott Jacobson last November (PRWeek, 1 December 2007 ).