Answering barchu while donning tefillin
Speaking is forbidden between donning the shel yad and the shel rosh. One should not even respond to a davar shebekedusha-e.g. yehe shmei rabah, kedusha or barchu. Someone who interrupts must recite an extra beracha on the shel rosh. According to the Sefardic custom he would have to recite the beracha of al mitzvas prior to putting on the shel rosh, whereas Ashkenazim add a second lehaniach in addition to al mitzvas. If the interruption was for a davar shebekedusha, some poskim hold that even Ashkenazim recite only al mitzvas on the shel rosh. Speech pertaining to the tefillin is not considered an interruption, but should be avoided unless absolutely necessary.
(סעיף י, ס"ק לד ו־לו, וביה"ל ד"ה אם)