You will not be able to use this as an option if all of your contacts are currently being stored on your
SIM card. Only a contacts name and number can be stored on a
SIM card. To use the phone to store the contacts instead of the
sim card: Press Menu Key - Address Book - then options - setup - Default storage - (choose phone).
Now you should be able to go in and edit your contacts and assign ringtones to them: Hit menu - Address Book - scroll to address book entry - press options - select edit contact (see if there is a ringer option here that will let you pick out ringtones for your contacts).