the present invention allows a mobile station to receive a command entered by a user for selecting among a plurality of possible values for an authentication key stored in its memory , including a default value , a random value and a custom value for the a - key . in response to the command , the mobile station can set the a - key in its memory to the default value or to an internally generated or previously stored random value , or to the custom value entered by the user . the command , whether for selecting the default , random or custom a - key value , may be entered during number assignment module programming of the mobile station .