when a cellular telephone is powered down , it sends a power - down message to a network it was registered with . if the cellular telephone does not receive an acknowledgement of the power - down message from the network , a flag is set in the cellular telephone . the power - down sequence is concluded by the cellular telephone , and the cellular telephone is shut off or put into a reduced power consumption state . when the telephone is turned back on the flag is evaluated to see if it was set during the course of powering down . the status of the flag when the phone is powered up is then used by the cellular telephone to determine whether it should make a network access request to a network the telephone was registered to , during the power - up operation in order to cause that cellular network to update its database regarding the whereabouts of the powered - up cellular telephone .