The SIM/USIM card typically only has storage for a fixed number of text messages (typically 20-40). When the phone wishes to store a text message, it first checks to see if there is space on the card. If there is, the message is stored on the card. If there isn't, older phones will ask the ask to free up space by deleting text messages manually.

Today's phones will instead store text messages in the mobile phone memory, but keep the newest text messages on the card, which is of course still useful.

When text messages stored on the SIM/USIM are deleted, they are usually flagged as deleted; the message itself (content, originating number, date/time received etc.) remains intact. USIMdetective is able to retrieve this information, but only if a newer text message has not already overwritten this data (remember that as far as the phone is concerned, this space is free for another message. In that event, the content of the previous message is lost forever.

USIMdetective is not able to read text messages (or any information for that matter) stored in the mobile phone - it only has access to SIM/USIM data.