Historical clue formulations

• Mask feature — 2011-06-05
• It lets you see who's calling — 2009-06-05
• Caller ID aid? — 2006-06-10
• Mask opening — 2004-10-12
• Circular opening — 2003-09-06
• Mask feature — 2001-04-11
• Socket — 2000-07-30