Can someone be a liable witness if they listened to a conversation via speakerphone (without the other party’s knowledge)? Can this “witness” testify to what was said by the party on the other end? How could they prove whoever is on the other end of the conversation is really who they “claim” they are? Would this be considered hearsay?
Yes the communication you are referring to is hearsay as one can not testify to what another person said outside of court. There are exceptions to this rule but this communication does not fit those exceptions.