Prepaid phone cards are convenient and can save you money. But according to the Federal Trade Commission, some complaints are becoming evident as these cards increase in popularity.

For example, some cards charge you even when your call does not go through. This one is a big surprise to consumers who are charged for calls that result in a busy signal. Check the fine print for this sneaky tactic.

The sign on the card dispenser says five cents per minute but when you purchase the card you find that you are charged 49 cents every time you make a phone call…another one to avoid. Then there are the companies that go out of business and leave consumers holding worthless prepaid phone cards. It’s hard to tell who will be around to provide the service for their cards.

Hidden expiration dates are another common frustration for consumers who suddenly find their cards are worthless even though they still have unused time.

Awareness of common complaints may help you avoid costly mistakes.