I'm currently travelling while working for ReignDesign, and my latest destination is the United States. I have an unlocked iPhone 4 (bought in Hong Kong). In Australia and New Zealand I was able to buy a Vodafone micro-sim, slip it into my phone, and have phone and 3G data service on a pre-paid basis. In the US, it's not so simple, for starters:
- Not all carriers are compatible with the GSM network used on the iPhone
- Most people are on contract: prepaid is much rarer than in other countries. If you're only in the US for a couple of weeks, a contract isn't practical.
- Most SIMs are regular size, not micro-SIMs
So, how did I get things working:
1. At an AT&T store, purchase a GoPhone prepaid SIM. This costs $15. Each day you use phone or SMS, costs $2, but you can make as many phone calls and SMS (even international SMS) within the day as you want. You may want to add an additional $20 credit at the store (see step 4).
2. Chop down the SIM into a micro-SIM. This is scarier than it sounds. I downloaded and printed a paper template from this page, then used scissors to trim down the SIM.
3. Insert your new micro-SIM into your iPhone (use a paperclip or pin to open the SIM slot). Verify you are able to make a call. The iPhone should show your balance.
4. To get 3G data, you need to subscribe to a data package. For $19.99 you can add 100MB of data. You can pay for this on the AT&T website (though this seems to require a US credit card, so borrow a friend's, or do this at the AT&T store)
5. Your phone will still give an error message "Could Not Activate Cellular Data Network: You Are Not Subscribed to a Cellular Data Service" when trying to use data services. You need to change the APN settings, which Apple helpfully don't provide a user interface for. To fix this, get on a Wifi network, go to the unlockit nz website, choose "Custom APN", then "US - AT&T", then install the profile.
And you're done!
Let me know if you have any problems following these instructions! Note this will likely only work for unlocked, but unjailbroken phones.