Note that in general data is trickier than voice, and many prepaid SIM cards will not work and you may have to make a few setting changes to your phone.

FORBES: Unlocking Your iPhone Could Save You Big, At Home And Overseas UPDATE

Once you unlock your phone you can insert almost any SIM card you want (the iPhone3 and 4 series take a different size SIM so be careful), foreign or domestic, prepaid or not.

FORBES: Unlocking Your iPhone Could Save You Big, At Home And Overseas UPDATE

Much like it did on the Motorola Photon Q, Sprint has elected to keep its carrier-specific branding off of the device (it's doing this so the handset can make its way to the network's prepaid brands with relative ease), but we were disappointed to see that the SIM card is once again embedded and not accessible without taking the entire phone apart.

ENGADGET: LG Optimus G for Sprint hands-on (update: video) Mobile

If you have a GSM phone, you can get cheaper service by swapping out a small card located in the back of your phone and replacing it with a local version. (This won't work on CDMA phones, as they don't have these cards.) Called "SIM" cards, they're sold in phone shops and can be purchased prepaid.

FORBES: How To Call Home From Outside The U.S.