Suppose you went into a convenience store to buy a container of milk for $2, but didn't know whether the clerk behind the counter was going to give you a pint or a gallon.

That's what it's like for many people buying prepaid phone cards for international calls these days, according to a Consumer Reports study. People trying to reach friends and family overseas spend billions each year on these prepaid cards, costing $2, $5, $10 or more. But often they don't know how much time that will yield on the phone.

For this project, which was funded by a grant from the Attorney General's Office in New York State, we bought more than 130 cards all around the state and then analyzed rates and fees.

We found the rates were usually not disclosed at point of sale or on the card, and numerous fees were listed on the back. Many had weekly maintenance fees, and fees for calls to cell phones. Many also said rates and fees were subject to change without notice.

With a subject this confusing, we explored what the pitfalls are in using prepaid cards and how you can get your money's worth.

To help, we created a tip sheet in eight languages, since many cards are aimed at immigrants to the United States. The languages include Arabic, Chinese, English, French, Hindi, Russian, Spanish, and Urdu.

For our full report see Shop wisely when buying prepaid phone cards.