Kitzur Shulchan Aruch 20:1 (yonanewman.org)
When the chazan steps (back) after (completing) the quiet (individual) prayers, he should stand still for the amount of time (it takes one) to walk four cubits. He returns to his place, and says quietly ''G-d, open my lips...'' , and starts aloud ''Blessed are You...'' Everyone should be careful to be quiet now, and listen carefully, and concentrate on what he is saying. They should say ''Blessed is He and blessed is His name'' and ''Amen'' to each blessing, as it is fitting. It is even forbidden to study during the chazan's repetition of the (Shemoneh Esreh) prayers. Needless to say, one must be careful not to indulge in idle talk. If it is easy for one to do so, 1 one should stand as if one is praying oneself the Shemoneh Esreh, or with closed eyes, or follow in the Siddur what the chazan is saying. Some have the custom, that after Kedusha, they remove the tefillin of Rashi 2 and put on the tefillin of Rabbenu Tam, 3 (but what) they do is not the law.
(bold is from me, the bold answers the question)