If a person is standing outside a synagogue and hears Kaddish, kedusha, Barechu or the Yag Middot (Thirteen Attributes of Mercy), he should answer the proper response, even though he is not situated inside the synagogue. Even if there is something in between him and the synagogue – such as a non-Jew, an idolatrous object, or even something unclean – as long as he does not see that person or object, he should answer to the recitation. This applies as well to somebody who hears these recitations via telephone. For example, if a person is ill and unable to come to the synagogue, and he "dials in" so he could hear Kaddish, kedusha and Barechu, he may and should answer the appropriate responses.
Likewise, if a person listens to a live broadcast of a Shiur via telephone, radio or a live feed over the internet, and Kaddish is recited after the Shiur, he should respond to the Kaddish. Since the person hears Kaddish recited live, he should answer. Of course, one does not respond if he hears a recording of Kaddish or of a Beracha. One responds only if he hears the Beracha or Kaddish at the time it is recited, such as via telephone or a live broadcast.
One should not answer "Amen" to a Beracha he hears via telephone or broadcast if he should have recited the Beracha himself. For example, before a person dons his Tallit, he should not call his friend in the synagogue, have him recite the Beracha on his behalf, and then answer "Amen." If two people are together in the synagogue and don their Tallit at the same time, then one can listen to the Beracha recited by the other and then answer "Amen," if he prefers not to recite the Beracha himself. However, this should not be done via telephone; a person donning his Tallit alone at home should not ask somebody else to recite the Beracha for him over the phone.
Summary: One who hears Kaddish, kedusha, Barechu, Yag Middot or a Beracha should answer with the proper response even though he is not in the synagogue, and even if he hears the recitation via telephone or a live broadcast. However, one should not have his friend recite for him via telephone a Beracha which he should recite himself, such as the Beracha over the Tallit.