If the SMS messages are saved on the SIM card it's very easy, use a tool like CHIPDRIVER or , to view the haw SMS data, change the header to mark it as current instead of deleted.
http://3d2f.com/tags/undelete/sms/messages/
From a mobile phones internal memory, this is a little tricky, there are some techniques you can use, such as service boxes and/or Flasher boxes (e.g. SHU Box). These will do direct memory reads from the phone, but you will need to work out how to interpret the data.


