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This is also called a "service charge" or "maintenance fee". It is a charge that is deducted from the value of your card periodically after your first call. Some cards have no maintenance fee. The ones that do will charge this fee either weekly, semi-monthly (twice per month), or monthly. The effect is to reduce the value of your card (and the remaining minutes) even when you are not using the card.
This is just another way to take your money. The fees vary widely. If you don't intend to use all the minutes on your card in a short time period, this fee can really hurt you.
Read the fine print! If you don't find it there, it could be anything!