What exactly do you mean by unlocking a phone or smartphone? Well, when you buy one, you get it with a network carrier serving it. But if you want to use some other network carrier, then you would need to unlock your smartphone. Here is a step by step guide to unlock any HTC smartphone for free.
Get IMEI Number Of Your HTC Smartphone
STEP 1: Call your carrier and ask for the MEP Code and they would ask for the IMEI number, so give it.
STEP 2: In order to know your IMEI number, type *#06# on your HTC smartphone and it will pop up the IMEI number.
Unlock HTC Smartphone
STEP 1: Now, switch off your HTC phone and switch it on by either removing the SIM or by inserting an unaccepted SIM.
STEP 2: It wont get switched ON, instead it would ask you to enter the unlock code. Enter the MEP code that you have got from your network carrier.
Now the HTC phone will reboot automatically and that is it, you are done with it. This is how you can unlock any HTC smartphone.