Don't have manual and phone shows "SENT" and ext #? (by 2 users)

I tried to call someone and they were on the phone, busy. So I tried to hit MESSAGE to leave one, but nothing happened. Then when I hang up the phone, the display screen shows "SENT" and the person's ext # I just tried to leave a message with... how do I get that "SENT" message to go away? Or did I leave a message without knowing it, and when that extension checks their messages, my message will disappear?