Some calling cards are for a one-time use, while others are rechargeable. A rechargeable prepaid phone card is convenient because you do not need to buy a new one when you run out of credit. You can follow the instructions provided on the card to add more money at any time.
When buying prepaid phone cards, compare the areas of coverage on different cards. Some cards can only be used domestically, while others can be used internationally. Make sure the calling card you buy covers the specific region you will be calling.