Here is a system that has worked for me in the USA on an iPhone 4 and an iPhone 5 in July 2012 and February 2013. Both phones were not locked to any particular carrier but they were not jailbroken.
- Visit an AT&T store or otherwise get a GoPhone SIM that fits your phone (micro sim or nano sim). Choose whatever rate plan suits your needs. I’m partial to the $25 for 30 days unlimited texts and 250 minutes. Note that the salesperson will almost certainly tell you that you won’t be able to get data on this plan – they are wrong just say ok!
- Connect your phone to wifi somewhere (Starbucks if all else fails!) Visit unlockit.co.nz through your phone’s browser. This site is going to make twiddling your internet settings a lot easier – it doesn’t jailbreak or carrier-unlock your phone it just changes internet access settings in a simple way. Choose Create APN, “USA” and “GoPhone”. Press “Install” to install the settings.
- Set up a PIN for your account by getting it texted to the phone. You can choose “Forgot Password” from the GoPhone login page to set it up.
- Sign in to your GoPhone account and add a data package (200MB for 30 days for $15 for light usage or 1GB for 30 days for $25 for heavier usage). You’ll need a credit/debit card to do this entirely online or you can use a refill card you bought in a store.
And that’s it. You may need to remove the APN settings that you installed when/if you leave the US.