Why is my BlackBerry losing its call logs or message logs?

Having a large amount of images, ringtones and or games stored in the device may reduce available free memory. If the BlackBerry Pearl’s memory is at or near capacity the handheld will begin to delete the oldest Call Log and Text Message (SMS) entries in an effort to free up memory space. In order to prevent this, free memory must be increased. NOTE: Saved messages will not be deleted.

(Thanks to BobRoss @ BlackBerryForums.com for collecting this information)

Determine the Amount of Free Memory on the BlackBerry

• On your BlackBerry, go to Options > Status. That will show your amount of free memory.
• If the amount of free memory available on the handheld is 0, the amount of free memory must be increased to prevent the handheld from auto deleting call log entries and messages.

Suggested Ways to Increase Free Memory

   Reduce Allocated Media Memory
      1. Scroll to and select the Media icon.
      2. Select the Pictures icon.
      3. Select the Device Memory icon.
      4. Press the Menu key, select Options.
      5. Change the Device Memory Limit to 12MB.
      6. Press the Menu key, select Save.
      7. Hard Reset the BlackBerry.
   Reduce Calendar Keep Appointments to 60 days or less
      1. Scroll to and select the Calendar icon.
      2. Press the Menu key, select Options.
      3. Scroll to and select Keep Appointments.
      4. Select the desired amount of time to keep appointments.
      5. Press the Menu key, select Save.
      6. Hard Reset the BlackBerry.
   Reduce the amount of time Messages are kept on the handheld.
      1. Scroll to and select the Messages icon.
      2. Press the Menu key, select Options.
      3. Select General Options.
      4. Scroll to and select Keep Messages.
      5. Select the desired amount of time to keep appointments.
      6. Press the Menu key, select Save.
      7. Hard Reset the BlackBerry.
   Clear Cache and Cookies
      1. Scroll to and select the Browser or Media Net icon.
      2. Press the Menu key.
      3. Select Options.
      4. Select Cache Operations.
      5. Select Clear History.
      6. Note: Clear History will no longer appear once it has been selected.
      7. Hard Reset the BlackBerry.
   Delete Unused Languages
   Note: If deleting both the unused languages and the sample video wait to restart the device until both processes have been completed
      1. Scroll to and select the Settings icon.
      2. Select the Options icon.
      3. Select Advanced Options.
      4. Select Applications.
      5. Scroll to and highlight each of the unused languages under the Input Support and Language and Input Support subheadings.
      6. Press the Menu key.
      7. Select Delete.
      8. Select Delete.
      9. When prompted to Restart select No until all unused languages are deleted.
     10. NOTE:Repeat steps 4-8 until all unused languages are deleted then select Yes when prompted to restart.
     11. Hard Reset the BlackBerry.
   Delete Sample Video
   Note: If deleting both the unused languages and the sample video wait to restart the device until both processes have been completed. The sample video is the pre-installed video for the blackberry and is ~2.2MB in size.
      1. Scroll to and select the Settings icon.
      2. Select the Options icon.
      3. Select Advanced Options.
      4. Select Applications.
      5. Scroll to and highlight "Blackberry Sample Video".
      6. Press the Menu key.
      7. Select Delete.
      8. Select Delete.
      9. When prompted to Restart select Yes.
     10. Hard Reset the BlackBerry.
   Change Call Log Options
      1. Scroll to and select the Phone icon.
      2. Press the Menu key.
      3. Select Options.
      4. Select Call Logging.
      5. Select either Missed Calls or None.
      6. Hard Reset the BlackBerry.