When traveling you might find yourself in the situation that you get a new SIM card into your iPhone and it’ll start and do it’s automatic magic for you. And eventually you well end up with the right provider settings by default.
But there are some cases when it picks the wrong provider settings. Like in my case. It picked NTT docomo in Japan with the default NTT docomo settings. In my case I was using a reseller for NTT (as described here) and that demanded different provider settings to work.
Unfortunately in all it’s wisdom the iPhone did not allow me to set the carrier settings. It just displayed the “Automatic” choice. So I went to the APN Changer website, entered the settings and installed a custom provider setting to my device. This works without any Jailbreak with iPhones without SIM Lock.