It depends on what you wish to do and why. If it is to block all calls except say internal

Create a CSS and assign it to the phone which block every call except internal and inform your users of Extension Mobility and let them over ride the previous CSS -internal only by logging on .

FAC codes- depending on the CCM level - FAC is used for external calls or calls via Route Patterns, again internal calls are OK byut to make an external call they need to enter a FAC- - FAC tend to be a single code which you give to lots of people