Using a public telephone is perfectly easy just about anywhere in Spain. To use one, you either have a choice of a coin-operated phone or one using a prepaid phone card. Phone cards can be bought at the any post office, tobbaconists (the ones with the Tabacos since out front), and newsagents.
To use a coin-operated telephone in Spain, simply pick up the receiver and put in a euro coin. Wait until you hear the dial tone then add more money and dial the number you are calling. If you don’t use all the money you put in, you will get a refund, but only to the nearest round number of coin you inserted. That’s why using euro coins is not usually recommended. If the call eats just a few cents into it, you won’t get the remainder returned.
To use a pre-paid phone card in a public phone, of course, just pick up the receiver and dial the access number. When it connects, key in your PIN number and then dial the phone number you’re calling.
Public phones in Spain usually also have instructions in English and, unlike many other countries, where so many of the public phones are vandalized, Spain has barely any problems with this. So, you should rarely experience a situation where you can’t find a phone that works.