A thing I don't like about Signal is that in principle if someone messages you by username they can't get your phone number, but the interface doesn't indicate at all. Meanwhile, if you get cold called by someone it doesn't tell you if they messaged by username or by phone number, and it's not clear by pressing "Accept" whether or not you're transmitting to them your username (if they messaged by phone#) or the phone# (if they messaged by username)… or how to prevent this.