Our group has been using or miss using the Message spell. after reading several of the posts. Instead of using message spell as a walkie-talkie, they have used it as a speaker phone. The question came up does the group hear everything or only what is a directed whisper like a text message.
I think the answer is as follows:
The caster, must initiate the conversation. if the caster is speaking to an alternative person, no other speaking is overheard if not designated as a whispered message recipient . the receiver must reply with in one round.