1 and 2) You do not have to unlock it. Just get the smaller SIM card from Straight Talk. It works with the 900. All you have to do is edit the APN. If you go to Settings, click on "Cellular". There is a button that says "Edit APN." Under "APN," enter "att.mvno" All other fields leave blank. Your data will work wonderfully. Do note though that MMS will NOT work.
Originally Posted by mchakri001
EDIT: If the iPhone you have uses the smaller SIM from Straight Talk already, that SIM will work just fine with the 900.
EDIT AGAIN: If you need any other info regarding the phone and Straight Talk, message me and I would be glad to help with it!