The SIM 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 SIM. If there is, the message is stored on the SIM. 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 SIM, which is of course still useful.

When text messages stored on the SIM are deleted, they are usually flagged as deleted; the message itself (content, originating number, date/time received etc.) remains intact. SIMdetective 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.

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