Blowing Shofar during Shemoneh Esrei - Igrot Moshe Orach Chayim 1:173

There is a practice to blow shofar not only before שמונה עשרה and not only during חזרת הש"ץ, but also during the silent שמונה עשרה. In this practice, people reach a certain point in their davening, then stop and wait for the shofar to be blown, then resume their silent prayer. Rav Dovid Feinstein asked his father what the law is regarding those individuals who daven slower and thus are still in the middle of a bracha when the shofar is blown. Is their pause in order to listen to the shofar and thus fulfill the mitzva considered to be an interruption in their davening?

Rav Moshe answered that there is no problem of הפסק, interruption, for two main reasons. First, even though we have a principle of שומע כעונה, under which a person is considered to have said a bracha merely by listening to another recite it, in this case the person is not considered to have said anything or even to have blown the shofar. He is merely listening to the sound and thereby fulfilling his obligation. Mere listening does not rise to the level of interruption.

Second, Rav Moshe reasons that the entire silent שמונה עשרה is considered to be an appropriate place for the blowing of the shofar, and thus wherever in davening one is up to when the shofar is blown, that is a suitable spot for him to hear it, and thus there is no הפסק issue.