I had the same problem about a week ago. I called t mobile they were no help at all. I was searching through the phone and found the i had no memory. I messed around with all the options nothing worked. You have to delete all your and your browser history. You have to go to this
1. go into your browser
2. press the menu key which brings up 3 options 1)browser 2) general and 3) cache.
3. open the cache operations
4. clear history, content cache, and the cookie cache.
After i did this i got memory back and my call log and messages were not deleted anymore. I found that you have to do this atleast once every two weeks or depending on how much ou use your browser.
Please let me know if this workes for you?