Don't buy the official San Francisco calling card from the tourist office - its a rip-off. Every time you use it, even if you hang up before connection or if your call is unanswered you pay a connection fee of 59/99 cents. There is also a 65 cent fee for calling from public phones.
I have lost half my credit just by trying to reach somebody, without talking a single second...

Try http://lowestrates.pushline.com. They offer a good selection of calling cards and you can search for the best rate to where you are calling to and from.