The leading poskim are divided on hearing havdala by telephone, hearing Megillat Esther by telephone, and other mitsvot that depend on hearing.
The disagreement among the posekim revolves around divergent understandings of the nature of hearing. This means that the concept of “hearing” must be clarified before addressing the halachic status of hearing something by telephone or VOIP.
A source sheet in both WORD format and pdf is available through the links. In class we will translate and explain the sources.