<!-- from fixya rte -->How to unlock a firefly t100 - FireFly Cell Phones

Instructions

• 1 Press and hold down the lock button on the Firefly phone until the screen says "Enter Pin."
• 2 Press the "Enter" button to begin entering the PIN number.
• 3 Press the right arrow button once. You will see the number one.
  
• 4 Press "OK." You will see an asterisk on the top line.
• 5 Press the right arrow button twice until you see the number 2.
• 6 Press "OK." You'll see a second asterisk symbol on the top line.
• 7 Press the right arrow button three times until you see the number three.
• 8 Press "OK." You'll see a third asterisk symbol on the top line.
• 9 Press the right arrow button four times until you see the number four.
• 10 Press "OK." You'll see a fourth asterisk symbol on the top line. The phone will unlock.
• 
  Read more: http://www.ehow.com/how_6361971_unlock-firefly-cell-phone.html#ixzz309pjkqds

Use unlock code to unlock your Sim in Samsung T100. Get the code from the site Mobile-Unlocker.com and unlock it using unlocking instructions which is given in the site itself.

Try country code here http://www.theunlockarena.com/sony-ericsson-t100/rs13wp9/

I am facing the same issue and does not seem to be a locking problem and firefly customer care has confirmed their phones are not network locked. However I did notice the issue could be due to different sim designs. The firefly sim design (the gold printed circuit at the back) seems to be larger than the airtel sim that I am trying to use on the firefly phone. I am planning to check with airtel if they can provide a sim of a different design.

Im sorry did you resolved your problem?
I have same problem (((?

contact AT&T customer service @ 1800 331-0500 and ask for an subsidy unlock code for the phone be ready to give the IMEI to Customer Service

YYou must insert a sim card that is not Cingulat/ATT brandeed and it will say enter special code Now! or Subsity code..
thnk u...

With ATT sim in phone, Power on & wait to initialize.
Then, press & hold the green key until the "Enter Pin Screen".
Then, press red button 5 times until the exit is displayed.
Then press & hold the phonebook key til the "unlock code screen".
Press the phonebook key once then. To enter the code you use the green & red keys to scroll to the proper numbers to enter 6 digit code.
You scroll to the proper number and press the phonebook key at the number you select until the entire code is entered.
Once done, you will scroll to the "done" and press the phonebook key once.