How to unlock a "network sim" in order to use a Android phone?
You'll need to purchase an unlock code.

You will probably find a number of companies via google a I have used a couple, which I'm not sure I'm allowed to post here so I recommend you visit Modaco, the site run by Paul O'Brien of custom rom and rooting fame.

Click the Android part of the site and at the bottom of the site or via a web banner to the right, you'll see one of their official sponsors that does HTC codes for $20.