If your wanting to block your calling I.D when dialling out from your Cisco 504G phone you can do this two ways.
Number one is by pressing the feature key that has a piece of paper with one corner folded over that is located to the right of the numerical keypad. Select the preference options from the available options. Using the directional keypad scroll through the menu and highlight Block caller ID, when this option is highlighted press the edit key. You will then have the option to choose either yes block caller ID or no caller ID is available. Once you have made your selection press ok and the save.
Number two is by entering the digits of *67 followed by the number you want to call, The *67 feature code will block your calling I.D.
I would always test this before making calls.