Surcharges are simply calculated as a percentage of the total cost of a phone call.
The surcharge is added to the cost of each call much like a "tax," although the carriers typically do not call this a tax.
For example, if the stated rate is 5 cents per minute, with a 10% surcharge, then the effective rate is 10% higher, or 5.5 cents ( 5 cents x 1.1 = 5.5 cents).
Surcharges are deducted from the remaining balance on your pin at the end of each call. For most cards, if you use up the card balance during your call, then you avoid the surcharge since it cannot be deducted from a pin with a zero balance.
Note: If you prefer cards without surcharges, do a ZapTel rate search and in Advanced Search Options select the Filter to "Show only flat rate (no-fee) cards."