Select a carrier in the settings!

[eff pc]

Ok so I realized after I saw this that I should have taken screenshots but I went into the settings just now and I'm assuming my phone glitched and under "Location Services" was an option for "Carriers". I clicked on it and inside this menu that I have never seen before was a toggle switch labelled "Automatic" and it was switched "On". I switched it to "Off and below popped up "Searching" however nothing ever showed up. I went back to the main Settings page and it was gone. Occasionally my phone will loose signal and display "Searching..." so this may have been why it glitched since I noticed it was doing this when I saw the menu however when I went back my phone had connected to the network again. I have an AT&T iPhone 4S BTW. If it happens again I will get screen shots. Is this a menu on the unlocked iPhone by chance? Maybe you will be able to switch carriers in the future? I don't know maybe someone else can shed some light.

 

[McGiord]

That is also shown when you are roaming, so it is nothing exclusive for unlocked iPhones.

 

[PaulyD]

I have always had this on my UK iPhone 4 and now 4S.

Use it to select T-Mobile (I'm on Orange) so go onto the mast shared network. Get a LOT better 3G coverage from doing so

 

[Daveoc64]

This is a standard feature of GSM/UMTS cell phones.

It allows you to choose a specific network to connect to, or you can use the Automatic option to let the phone do it for you.

The iPhone has an annoying quirk where it will pop up a notification if your chosen network's signal is lost, meaning that it's quite annoying to actually choose a network manually.

The menu should list all GSM/UMTS networks available in your current location, but you'll only be able to connect if the network allows you to. In most situations, that'll only be to the network that you subscribe to.