Date Entered on the Date Completed Field on Phone Calls is not Retained After Saving

Navigate to Activities > Scheduling > Phone Calls.

To save the value users enter in the Date Completed field, users need to set the Status of the phone record to Completed or click the link Completed link in the list of phone calls (under Mark column). 

The current date will automatically populate or users can manually set the date after changing the status to Completed.