For a reliable unlock solution use a website which has been around since 2006. There are many scam websites so be very careful!
For example the PROunlocking site mentioned before is from a company whom spit out 'unlock websites' by the hundreds and just try mislead customers as much as possible. They provide zero customer support (I know because we often get dissapointed customers from them with still locked phones).
There are plenty of reliable sites just google around, or of course use ours (biased) at http://www.unlocksamsungonline.com/unlocksamsung/E2121B
note that for unlocking by cable you need a SPECIAL FTDI usb cable and it will NOT work with original USB cable. For this reason I would recommend unlock by IMEI (http://www.unlocksamsungonline.com/samsung-E2121B-unlock-codes/