Here are the steps you'll need to take to ensure you'll be able to make phone calls when the plane lands:
Before you leave, check with your carrier.
Tell them you're going overseas and want the ability to make calls. Ask if they have any special international calling plans that you can sign up for. If they say their network doesn't support international roaming, you might need to rent a phone.
If you're lucky enough to have a GSM carrier, make sure your phone has a SIM card.
You can easily find this out by checking your manual, and if you've tossed the manual just look under the battery. You won't need to disassemble your phone. If you have a phone that does not have a removable battery, it probably doesn't have a SIM card either (although some higher-end phones such as the Handspring Treo 600 don't have a removable battery, but do have a removable SIM card).
Get a cheap, unlocked GSM phone with a SIM card.
If you're on a GSM network, but don't have a phone that supports SIM cards, head to your carrier's retail outlet and pick up a cheap phone that has one. Most models manufactured in the last few years have this technology built in.
Rent a phone.
If you're on a mobile carrier that isn't using the GSM network, then you'll need to rent a phone (or use pay phones or phone cards). The charges will depend on the call usage and length of rental. Usually, if the daily calls average more than $10 U.S. per day, the rental is free. Otherwise, expect a rental charge of about $1.50 U.S. per day.
Preparation by destinationsome examples:
TRAVEL TO EUROPE, CHINA, HONG KONG, SOUTH AMERICA
Americans traveling to countries that use 220v electricity need to "step down" the current to match their 110v appliances using a converter like the one shown here.
Bring a GSM phone that works on GSM 900 and 1800 systems.
Unlock your phone if you want to use a local SIM card.
Bring plug adapters (for two- and three-prong devices) and an adapter for converting 220v to 110v.
Make sure your laptop's power cable and your cell phone charger work on a 220-volt system.
TRAVEL TO USA, CANADA, BERMUDA, BAHAMAS, MEXICO
Make sure your GSM phone is a three-band phone that works on GSM 1900 systems.
Bring U.S. plug adapters and a voltage adapter (to convert electricity to 110v).
Make sure your laptop's power cable and your cell phone charger work on a 110v system.
TRAVEL TO JAPAN
Your cell phone might not work in Japan. You'll probably need to rent a Vodafone GSM 3G phone and check with your carrier to make sure it has a roaming agreement with Vodafone.
Bring plug adapters that reduce three prongs to two.
Consider a voltage adapter. Japan uses 100v versus the U.S. 110v. Many appliances work just fine when plugged into a Japanese outlet, but consider additional protection for sensitive appliances.