Beware of very low rates, particularly for international calls. They may indicate poor customer service. Some cards add a surcharge to the first minute of use. Others charge an activation fee for recharging cards. Check on expiration dates.
Most cards expire one year after first use.
Look for a toll-free customer service number. If the customer service number isn't toll-free or displayed, it may be difficult to contact the company if you have a problem with the card. A busy signal on the customer service line may be a tip off to a rip-off. Be sure the card comes with instructions that you understand. Otherwise, anyone who copies the PIN can use the phone time you've already paid for.