I have bought a used Xperia X10 for development. It is locked to a network provider, and I need to unlock it. It does not show an option for a passcode or unlock code.
Is there any other way to unlock the phone?
It doesn't look like there's any reliable way to unlock it without the unlock code. You should call the carrier and ask them to help you unlock it. Most will give you the code if you are persistent enough, and apparently you can enter it by typing
*#06# on the dialer.
Each mobile phone has an unique unlock code.
You can buy the unique unlock code for your Sony Ericsson Xperia X10 mobile phone in Unlocking4U, and follow their free instructions to get free from the locked network.