The cards are called prepaid phone card or calling card. You can buy them in nearly every Supermarket chain or at gas stations or souvenir shops.
The value varies from about 10 Dollar to 50 Dollar and the issuer are most of the big telephone companies like ATT, MCI, etc. There are also cards available at drug stores like Walgreens or others. The price per minute differs from company to company and it is difficult to get a final clue how much a minute call to your country will cost. But the cards tell, how many national minutes (within the US) are on the card.
The cards have on their back the instructions, how to use the card, listed. You will see a toll-free access number that you'll call. Then a computer voice will ask for the access code which can be found on the back of the card. You will type in the access code and then you listen to the computer voice, which will ask for the country code and telephone number you wish to dial.
Assuming you want to make an international call, you have to dial
011 first, then enter your country code, then your area code and finally the phone number.
You have to know your country code.
Some examples can be seen on this page.
There are also rechargable calling cards that become very practical when you run out of minutes and you have immediate need to refill the card but no store is close by.
Then you can recharge those cards with your credit card by either calling the customer service or using a website.
One of the international companies in this market is the IDT Corporation, see WebLink below.
Otherwise, you don't need to worry, as said, calling cards can be bought in nearly every supermarket - just chosse how much you want to spend and pay - no credit cards risk, a simple transaction.
Be carefull when searching on the Internet for calling cards, especially cheap ones, you have to evaluate who's behind the offer, that means it's a market, which is not really transparent.
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