In Devarim, (28:10) the Torah tells Bnei Yisroel that the nations of the world will see Hashem upon you and will fear you. The gemara in Brachos (6a) says that this posuk is referring to the mitzvah of tefillin shel rosh.
In Shulchan Aruch Orach Chaim (25:9-10) we pasken that one may not say any words between the putting on the tefillin shel yad and the tefillin shel rosh. If a person hears kaddish or kedusha, he should not answer, but rather pause and listen and then finish putting on the shel rosh.
What happens if a person is between the tefillin shel yad and shel rosh and he hears someone else making a bracha on tefillin? May he answer amen as it is not a hefsek since it is part of tefillin? Is he not allowed to answer amen?
The Shailos Utshuvos Panim Meiros (1:59) writes that if a person hears the bracha of tefillin from someone else and he is between the shel yad and shel rosh, he may answer amen. This is despite the fact that one can’t be mafsik for kaddish and kedusha at that time. The reason for this is that saying amen on the bracha of tefillin is like part of his mitzvah, whereas answering for kaddish and kedusha is going off topic. The reason we are not mafsik between shel yad and shel rosh is based on the Ran in the end of Meseches Rosh Hashana in the name of the Reza who says despite the fact that shel yad and shel rosh are two different mitzvos; nevertheless, the posuk uses the terminology of “Vehayu” - they shall be. They both have to be put on without a hefsek and by answering amen to a bracha of tefillin, it would be considered being oseik in the mitzvah of tefillin. The Kitzur Shulchan Aruch (10) concurs and writes that if a person heard the bracha of tefillin from someone else while he is in between the shel yad and shel rosh, he may answer amen since it is part of one “Vehayu,” as it connects the shel yad to the shel rosh.
Reb Akiva Eiger in Shulcha Aruch (28:9) questions the Shailos U’teshuvos Panim Meiros based on what the Tur in Orach Chaim (61) says that one may answer amen on the birchas Krias Shema of the Shaliach Tzibbur and it is not considered a hefsek. The Tur says it is not any worse than if a person would give you Shalom Aleichem and you would be allowed to be mafsik to respond. We see that if this occurs in a place where one would not be allowed to answer to a Shalom Aleichem, a person would not be allowed to be mafsik by answering amen to the bracha. Since a person may not be mafsik even to answer Shalom Aleichem during the time between shel yadand shel rosh, it would stand to reason that it is prohibited to answer amen to someone else’s bracha on tefillin between the shel yadand shel rosh.
The Shailos U’teshuvos Devar Shmuel (142) says that answering amen to a bracha of tefillin between shel yad and shel rosh is a hefsek. He says it’s not comparable to a person who already washed for bread and he would be permitted to ask for salt and the like, and that is not considered a hefsek. The difference here is that regarding a seuda, you are asking for things that are needed for the meal; whereas regarding the tefillin, you are not saying something that is needed for your tefillin. It is therefore considered a hefsek.
The Mishna Berura (25) paskens like Reb Akiva Eiger that one may not answer amen since it would be a hefsek with the exception if a person is motzi you with the bracha of tefillin. Reb Moshe Feinstein in Igros Moshe (2:110) also paskens like Reb Akiva Eiger.
May we be zocheh to put on tefillin ke’Halacha.