[Question] How to remove simlock for my old iPhone 3G?

I'd like to give my old 3G to my parents, but run into the issue that itunes says their simcard isn't supported, which is true because the phone has never been unlocked for other simcards. I've been looking at the solution on the t-mobile website (which is my provider), and it gave me the strange answer where i had to keep my phone attached to itunes for 30 mins, then remove and reattach the cable - but that didn't work.

What are my options here? Can i use a tool to do that? I don't want to jailbreak it, just removing the simlock is what i'm after. Any suggestions? A question i have is: can i jailbreak it to remove the simlock, and then reinstall the official ios again?