Where I open the iPhone 3G to all networks?

Again, more specifics would help. How are you "opening it to all networks"? Do you mean you set it to notify you about all open networks? If that doesn't work, make sure the 3G network is indeed turned on. Settings>General>Network>Cellular Data>On. If you don't know what I'm talking about, then you probably didn't turn on the network search option in Settings>General>Networks>Wi-Fi Networks>Ask to Join Networks>On.

Hope that helps. If not, give some more info and I'll try again.