After trying to unlock my own SPV C600 (ORANGE) and succeding i found out that it was very vague. There were no good step by step guides on how to do this. And so i compiled a little Step By Step guide my self. It consist of 5 simple steps and everything that you will need is included in the attached file. Just read the howtounlock.txt and follow the instructions.
This might work on a C500, not sure can't really test, but you can try it, i think it is similar. I cannot garantie anything though, use it at own risk.
I did not write any programs in the guide, so all credits go out to those peaple not me. I just put it all together.