Depending upon the call type, the call log provides the following information about the last hundred calls on the phone:

Avaya J100 Series IP Phones store the call log in a file saved on the phone. J100 series phone software version 4.0.3 and later always encrypts the contents of the call log file.

If you downgrade the software of your phone to a version earlier than 4.0.3, you will lose the call log details. To retain the call log details, enable the offline call log feature in System Manager before downgrading the software.