A: Can you hear me?
B: Yes
B: What time is it?
A: ...
At the point that B has replied Yes, B knows that it can hear A and that it can send to A but it doesn't know that A can hear B. As long as A makes the first move in the rest of the conversation that's fine - the next message from A confirms that B's "Yes" was received, but if A has nothing to say then B has to send it's next query and hope that A received the Yes successfully. If it didn't then B thinks the connection is established but it actually hasn't been.
A: Can you hear me? B: Yes B: What time is it? A: ...
At the point that B has replied Yes, B knows that it can hear A and that it can send to A but it doesn't know that A can hear B. As long as A makes the first move in the rest of the conversation that's fine - the next message from A confirms that B's "Yes" was received, but if A has nothing to say then B has to send it's next query and hope that A received the Yes successfully. If it didn't then B thinks the connection is established but it actually hasn't been.