I always took this to actually be an instruction! telling the user to hang up, like the dial tone when on a landline when the other person hangs up (In the time it was made this makes sense: hear tone meant put the phone down as the other person has hung up).
Or maybe in deep space, considering that most messages aren't 2 way: you hang up to acknowledge receipt of the transmission so the relay station doesn't have to keep sending it. It could be as simple as marking an email as read (by ending) or leaving unread (by hitting cancel)
This is pure speculation but I always saw that message as an instruction to hang up and clear the line: which is how most landlines worked back then - if you don't hang up then no-one can call you till you do (or send you another message in this context)
It make sense with long range communications: if you received the message in tack then 'end transmission' (confirm), else 'send another copy' (cancel: or something). It is asking have you received the message intact AND its not been scrambled in transit --- maybe a question-mark at the end is missing but i think it is a question