Can you answer amen to a Baracha that was made over the telephone?
See which concludes that Rabbi Shlomo Zalman Aurbach Zatzal held it was not permissible.
yes and also ieh shemeh rabah, and I think 13 midot and kedusha also, not because you heard it (because you didn't) but because you know someone finished a beracha.
this would also apply in a place that people say amen loud before the hazan finishes the beracha, technically you didn't heard the ending but you can say amen because you know he did.
note that live radio would be the same
most poskim rule that one cannot fulfill an obligation (such as hearing Havdalah) by listening to the bracha on the phone. This is because a phone is a simulated sound and one does not actually hear the person who is reciting the bracha. That being the case, one would assume amen cannot be said in response to a bracha recited on Zoom or over the phone since the person did not hear the actual blessing.
However, it is clear from the Gemara that there are times when a person should answer "amen" without having heard the bracha. The Gemara (Sukkah 51b) relates that the shul in Alexandria was so large that much of the congregation could not hear the shliach tzibur. To assist the congregation, the gabbai would wave flags at the end of each bracha as a signal to answer amen. Based on this Gemara, the Shulchan Aruch OC 124:8 and later poskim ruled that one should answer amen even without hearing the bracha, provided the person knew which bracha was said and there was no intention to be yotzei with that bracha. For this reason, many contemporary poskim such as Rav Chaim Kanievsky, Rav Hershel Schachter and Rav Asher Weiss, have encouraged answering amen to brachos heard over the phone or through Zoom.
However, Rav Shlomo Zalman Auerbach (Minchas Shlomo 1:9) writes that one should not answer amen to a bracha heard on the phone, and he considers this an amen levatala (a wasted amen). He explains that in the shul in Alexandria everyone was together in one building, but if one is miles away there is no connection to the person who recited the bracha and amen cannot be said.
Also see two teshuvos of Rav Moshe Feinstein (Igros Moshe Orach Chaim 4:91-4; 2:108), and a teshuva of the Tzitz Eliezer (8:11), which rule that in extenuating circumstances one can be yotzei havdalah over the phone