If you are experiencing the following symptoms; your device may be low on memory.
Automatically deleted call log
Automatically deleted SMS Text Message or e-mail
All RIM BlackBerry devices with handheld software version 4.2 or later utilize Low Memory Manager (LMM). Once the device memory reaches a threshold of 400kb or less, LMM will automatically remove data in order of importance to free up device memory. Once the device has reached full capacity, the device will not be able to store any new data such as SMS text, call log entries, email, etc. This is a list of the items which may be deleted: