The Mishna Brurah 91:6 brings the Zohar that writes that one should cover one's eyes with their talis during shmoneh esrei . There are those who say that one should only do this when one isn't wearing teffilin since one should not cover the teffilin(shel rosh) completely. However, from a plain reading it seems like it should be done with or without teffilin.
There is a way to be mekayim both ideas of covering one's eyes and leaving teffilin exposed ,by covering one's eyes with the sides of the tallis and leaving teffilin exposed.
Rav Chaim Kanievsky held that when the MB wrote that one should cover their head and eyes with a talis it means when one isn't wearing teffilin. He also held that when the Arizal said to cover the teffilin it meant to cover just a bit of it ,but not all of it.
Text of MB:
משום הכון לקראת וגו' - איתא בזוהר ואתחנן מאן דקאים בצלותא בעי לכסויי רישיה ועינוי בגין דלא יסתכל בשכינתא. [והיינו בטלית של מצוה וכן נוהגין כהיום לכסות ראש עד עינים בטלית מצוה בשעת תפלת י"ח. פמ"ג] מאן דפקח עינוי בשעת צלותיה מקדים עליה מה"מ וכו' ועיין בסי' צ"ה במ"ב סק"ה
Text of Daas Noteh (Rav Chaim Kanievsky opinion):
