Stock firmware won't make any difference.
Think of a smartphone as a computer; there's the bios and then the operating system.
The bios contains system information and settings. The operating system is what you actually see and interact with once you've logged in. It contains the apps and media you use.
When you boot a computer it loads the bios, and the bios tells the computer where the operating system is, what kind of operating system it is, an then proceeds to load it.
Smartphones are the same sort of idea, except part of those system settings is the network lock. The only way to unlock it is to request a network unlock code from the carrier it was originally on, or to hack the phone yourself.
Unfortunately, no one on Tom's will help you hack the phone. I will direct you somewhere that will, however remember that it will void your warranty and any mistake you make could potentially turn the phone into a paperweight with a battery. If you still wish to go ahead with it, check out this forum for more help: http
/forum.xda-developers.com/