Are you saying that when you have a NEW record it shows something like "Call ID: (Autonumber)" and when you're on an existing record it shows something like "Call ID: 123456"? And that when you have a new record you want to hide it?
If that's the case, remember that autonumbers are nothing more than ID tags for records and should never been seen or manipulated by users at all. If you need a visible, meaningful Call ID, you should create a new non-autonumber field.
But to answer your question, on the form's ON CURRENT event, put:
Me.CallID.Visible = Not (Me.NewRecord)
Assuming your field is called CallID.
Michigan Dept. of Environmental Quality, Air Quality Division