Wednesday, June 27, 2012: Now unlocking SIM in your HTC phone no longer needs you to go through frustrating arithmetic calculations. With root exploit, you can do it yourself for free without depending on anybody by using the Bash Script, which automatically does the trick. If you don't want to SIM unlock your HTC device at all, then you can wait and use your phone for at least 90 days as after that T-Mobile and most other service providers will give customers an unlock. Alternatively, you can also buy a SIM unlock code for the HTC One S for $12.95 from HTC SIM Unlock.
You can wait. You can buy. But if you want to get it done by yourself, then fold your sleeves and follow the steps given below as mentioned by XDA Developers, to get HTC One S SIM unlock code for free with Bash Script.

1) Root your HTC One S

2) Download Bash Script zip file

3) Extract unlock.sh file from zip to /system/ folder on HTC One S

4) Execute the .sh file directly from root explorer or open terminal emulator and run: su/system/unlock.sh

5) SIM unlock the HTC One S

6) Once SIM unlock code displays on screen(also saved to /sdcard/sim_unlock_code.txt), write the number down and turn off your HTC One S

7) Remove the microSIM card from phone

8) Insert a microSIM card from another service provider and turn the HTC One S back on

9) When prompted, enter the SIM unlock code and its done!

Freedom! Use your HTC One S on any GSM service provider of your choice.