you are experiencing the notorious Pearl Memory Leak. all the Pearls have it, so its not an issue with only particular phones. basically, your device thinks its running out of memory, so it starts deleting stuff...namely, call logs and emails. to avoid this, try to free up as much internal storage as you can (i.e. delete the sample vid, delete language packages you don't use, etc.) and try to make a conscious effort of deleting text/emails/messages that you no longer need on your device.
also, resetting your device on a regular basis...perhaps every other day, right before you go to bed or something, would help. after you've freed up a bunch of storage, check your status to see how much free storage you have...then use the phone for a couple days, and you'll notice its going down (especially if you do a lot of emailing, etc.)...then reset the phone, and you'll see the number go back up. its wierd, and there has been no fix for it yet.
best thing is to install SoftReset (its a free app) that essentially does a battery-pull reset w/o you having to actually pull the battery.
hope this helps!!
deleting call log and emails 
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