Users simply insert a SIM card with credit to begin making calls at local rates.

ENGADGET: SpareOnePlus adds location tracking, audio alerts, keeps the 15-year life on a single AA battery eyes-on

Simply insert the SIM from your current phone into iPhone 3G and connect to iTunes 8 to complete activation.

ENGADGET: Apple now selling iPhone 3G unlocked in Hong Kong

Does it only work with the SIM it came with, or can you insert your existing SIM?

ENGADGET: We've got iPhones, what do you want to know?

SpareOne is SIM unlocked, so it can accept SIM cards regardless of the GSM operator, and micro-SIM adapters are also available for iPhone users to insert their iOS SIM.

ENGADGET: SpareOnePlus adds location tracking, audio alerts, keeps the 15-year life on a single AA battery eyes-on

You can insert your existing SIM, but you will still have to go through the iTunes activation process.

ENGADGET: We've got iPhones, what do you want to know?

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

And, in the few days I was using it, a little door covering the USB port fell off and the top cover, which I had removed to insert a cellular SIM card, kept coming loose.

WSJ: Two Screens Aren't Better Than One for Sony Tablet P

While you can always insert a local SIM card into your phone for use when you are in port, this will not help you if you are on a cruise ship at sea and out of range of a local carrier.

FORBES: Verizon Smartphones And Cruise Ships Don't Mix

Assuming that you have an unlocked iPhone 4 or 4S or Android GSM (which is usually unlocked when sold), you can insert a local SIM card and have local voice and data services at a fraction of the cost you would otherwise pay.

FORBES: How to Save Money on Mobile While Traveling in Canada