The gemorah in Sukkah daf 51b explains that, in the enormous shul in Alexandria, in order for everyone to know when to answer Amen when the shliach tzibbur (prayer leader, aka sha"tz) was davening, ...
It says in the Baal HaTanya's Hilchos Talmud Torah that one fulfills his obligation to learn Torah only if he says the words. Moreover, a Chavrusa fulfills his obligation only because Shomeya Keoneh. ...
May one be yotzai listening to havdala over the telephone?