Yes its true, you can charge your credit card to make a telphone call using some machines in some areas of Greece, mostly airports where they are often the only option.
Please read the instructions carefully and find out the rate that you will be charged as these may be exorbitant.
I dont know which company runs these type of credit card phones and believe that the cheapest way to call internationally is with a pre-paid international phone card which needs a special type of phone but are far more common than this type of credit card phone left.