I have worked with a bank in Boston and was attached with Visa Credit Cards. I hope my advice will be useful to some travelers here.
1. When you use your C.C far from your home (even next door state), the security software will automatically refuse to process the amount until you call them back. They think your card is stolen. There is a lot of scams and CC theft in Europe and North America especially. They will process it once they verify who you are over the phone. CC companies have toll-free numbers printed on the cards and accept reverse calls expenses. The CC people can process the transactions on the spot as soon as you call in with the problem. Retailers who are not trained will tell the card holder that the card was refused but it is a matter of security to protect both parties and he/she is supposed to call CC company to verify the card holder.
2. If you do not spend more than 200 dollars or so and suddenly charge 3000, your card will not accept this charge until you verify that it is you who is using it. Most people think that they are denied access or there is a problem with the CC, but it is for the benefit of a card holder and reduces fraud.
3. If you travel overseas or far from your home and expect to spend a lot of money at once, it is useful to call back customer service and let them know. The card might still need verification, but chances are the expenses will be accepted.
4. If there is a sign of Visa or MC at the ATM, then your card will be accepted automatically there. 99% of the problem is due to the fact that CC software thinks the card is stolen, if the expenses come from New York when the card was issued in Texas or Sydney. A simple call will process this transaction on the spot. Again, calling ina dvance will help eliminate this problem most of the time.
5. The best place to use CC in Japan is Japan Post ATM or Citibank ATMs. However, it is very inconvenient because ATM machines mostly work until 5 or 7 PM. If you really need to use it bad, then the best bet is using ATM's at the airports. they accept 24 hours and the exchange places also never close.
6. Japanese-issued CC are the worst. They hardly have any security software so if someone steals it, they can use it right away since it most retailers don't need to see your ID or make you sign for the slip in front of them as they check your signature. Most of the time, you don't need to sign it, the machine processes it automatically.
7. The worst places to use CC are at the gas stations and telephone booths. Many scammers use cameras nearby and also bribe poorly-paid cashiers to get the credit card info and make a fake CC. To avoid it, try to use cash as much as possible in these places. Any airport will have a bookstore where you can buy a telephone card to call overseas. Telephone companies in America also issues telephone cards and you can use it abroad. It works like a debit card, you can use toll-free operators in many countries and they connect your call anywhere. You get a regular telephone bill at home. I used this service before, but its more expensive than buying local pre-paid telephone cards at the airports or convenient stores.
8. Remember, if you didn't use your card, you are not liable for the expenses. However, after 30 days, you will be liable if you do not call back and refute the bill. Therefore, if you travel, check your credit cards online and make sure you have legitimate bills only showing and keep your statements secured.
9. If US government or UN imposes economic sanctions to countries such as Cuba, Iran,Burma or North Korea , chances are your card will not be acceptable there. I wouldn't travel to any country under UN embargo anyways.