OK I know this is probably a hot topic at present - but I have a Nokia 1200, the new sim arrived today with a letter that has a code to unlock the phone. It also has an unlock code on the sim card. Both codes are different. Now I put the new sim in, and it says sim not valid. I took it out tried again - the same. So i entered the code on the letter and pressed the handset button. Nothing happened. So I tried the code on the sim card - again nothing.
I have just been though this with a lot of PMs. My Nokia was locked to virgin network and new SIM won't work with it. Seems the older Nokias can't be unlocked and I need a new handset in order to use new SIM. The same happened when I tried to unlock my previous Nokia - it couldn't be done.