You buy phone cards to use in the phone boxes. There is a value that is only for non international calls but I forget which one. You reception should know
We found the cheapest way of phoning home was using the phonecards. A word of warning here, don't buy them in the lobby of the Rila hotel, they are almost twice the price of the shop down the road. Another thing, try to make it a landline you call, for some reason phoning a mobile at home munched the cards.....