I just put a net 10 sim in a att iphone 3gs and it works?

I didn't get the phone unlock and the iPhone worked l, why do everyone. Keep saying you have to unlock it because i didn't need minds unlocked. It made phone calls and the internet worked so can someone tell me why everyone. Saying unlock it. Its a net10 sim is att

I've read online somewhere that you don't need to unlock an AT&T cell phone to use a Net10 SIM card, or vice versa. In the UK, there's two SIM cards that are similar; you don't need to unlock a Virgin cell phone to use a T-Mobile SIM, or again, vice versa. Some networks obviously allow this and don't require an unlock.

The phone usually needs to be unlocked. The only reason that I can think of that your phone does not need to be unlocked is that the Net10 sim card is programmed to work with a AT&T phone.