is there a number you can dial before ringing anyone so as to make your number appear as 'private' on caller ID (I know here, if you dial '0197' before putting in the phone number it blocks caller ID; i found it in the info section of my phone book, can't remember where exactly sorry!), but as splitimage said it depends on who is operating the hotline and what the nature of the call is.
*just wanted to add that this even works when I call a friend at CPS using their main number; she never knows it's me calling coz it comes up private with her too. that surprised me heaps given where she works...