The phone is still locked you can't flash a nokia phone to unlock it.
Flashing the mobile need a little bit off previous experience.And sometimes flashing damages/crashes mobile or loss of data happens if not flashed properly.Facing network unlock/security reset problem it is very easy to go with unlock/master reset codes.You can get codes for your phone from the website unlock-free.com freely with instruction.
HTC Touch Diamond, Blackberry Pearl (non-3g), Nokia 6275i
I dunno, generally flashing it with stock Nokia (not the provider's) firmware will work. However, if you used your provider's firmware, you'll need to unlock it via. a code. There are plenty of sites online that can provide you with the unlock code for a small fee.