I have seen this occur whenever we retrograded the firmware on the phones, ie, take the phone firmware (flash) back to a previous level for some reason. Several monthe ago we reflashed 1700 phones with earlier firmware (9.02 from previous 9.1.1) only to take a severe beating from our users the following day. They not only lost their directories but also lost their call history & custom ringer settings and screen backgrounds as well. It was not a good day.
(For those interested in why we rolled back, 9.1.1 firmware did not correctly support the Electronic Hookswitch feature with the Jabra GO-6470 series bluetooth headsets, though the prior firmware did. Go figure)
Original MUG/NAMU Charter Member