You can use phone cards for either public coin pay phones or phones in homes. Your phone card you purchase has an access number on it which you dial and then enter the code number on the card and then dial the number you're calling - they're pretty easy as they all have instructions on them.

I've not used a public coin payphone for some time but I seem to remember that they accept quarters (25 cents) coins but they may accept other coins too. I've absolutely no idea how much a short phone call to the UK would cost but I'm pretty sure you'd need a fairly large bag of coins!