Is It Permissible to Answer Amen to a Blessing Over the Phone?

Greetings, If no more than the time of "toch k'dibur" (approximately one and a half seconds) elapses from the end of the blessing until you hear it, it is permissible to answer Amen. 

Sources: See the halachic clarification in Shut Yabia Omer Part 2 (Siman 68), which notes that one who listens to Selichot and the Thirteen Attributes live on the radio may answer Amen. Also see Shut Igrot Moshe, Orach Chaim Part 4 (Siman 91, subsection 4), Shut Minchat Shlomo Part 1 (2005 edition, Siman 9, note a, subsections 4-5), and the sefer Ashrei Ha'ish, Orach Chaim Part 1 (Chapter 10, paragraph 14). However, one should not answer Amen if more than the "toch k'dibur" time passes before hearing the blessing, as explained by the Rema in Orach Chaim (Siman 133, section 8) and the Mishnah Berurah (S.K. 34). The Mishnah Berurah there discusses that the measure of "toch k'dibur" is disputed among poskim; some hold it to be three words’ worth of time, others four. Indeed, the Mishnah Berurah (Siman 65, S.K. 12, Sha'ar HaTziyun 111, Siman 177 S.K. 44, Sha'ar HaTziyun 113, and Siman 182 S.K. 7) rules decisively that the time of "toch k'dibur" is defined as the time it takes a student to greet his teacher with the words "Shalom alecha Rabbi." The Gaon Rabbi Chaim Kanievsky, shlit"a, also writes this in Sha'ar Ha'ayin (page 395), and that is the accepted halacha. According to this, if more than one and a half seconds passes from the end of the blessing to the moment it is heard, then one should not answer Amen. Although it is permitted to respond Amen to a blessing over the phone, there is no obligation to do so. See Biur Halacha (Siman 316, section 2, the phrase "chayav"). 

With blessings, Hillel Meirs