You need to delete some of the applications, emails, txt or media files that you have save on your phone. The reason the phone is deleting your call logs & probably sms is because your do not have anymore internal application memory left. Purchase a micro-sd card and transfer all you multi-media media files to it like pics & vids. Then you need to go to setting or options from your home screen, advanced options, applications & delete any 3rd party application that you may have installed that's not being used. Hope that helps