Summary: You do say 'Amen' when listening only live through a telephone or simulcast (dont think the medium makes a difference). You however arent yotze a mitzvah if you do it this way.
Taken from here
Rabbi Eli Mansour
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
He doesnt mention the exact source, but from the mp3 of the class it sounds like he is reading from a sefer.