1)    One who hears the cantor from his own house when the cantor is saying Kaddish, Barechu or Kedushah in the synagogue, he may respond to all of them, as since the Divine presence rests on the quorum in the synagogue, even an iron wall cannot divide between it and anyone who wants to join himself with his Father in heaven who is resting there. (Shulchan Aruch and Mishne Berurah 55)

2)    Some say that this is only on condition that no dirty things or idols are in between him and the synagogue. (ibid). Some say that the law is that even in this case one can answer “Yehei Shemei etc.” and kedushah but not Barechu. (Mishne berurah ibid)