When a call doesn't connect, "did not answer" is the only thing displayed. On Messenger, when a call doesn't connect, "[name] is on another call," "[name] did not answer," and "couldn't connect" are all possible reasons for a call not connecting that are displayed to the user. This allows the caller to know whether the recipient actively rejected a call or simply is offline. That's useful to know whether it's polite for the caller to call again later after the call doesn't go through or whether the caller should just wait to be called back.