If you look hard enough you can find phone cards in almost any denomination.
They have cards for $10, $20, $30, and they even have dollar phone cards. If you are looking for a calling card you can find them in almost any amount you want.
Now they even have rechargeable phone cards. There are internet site on the web that will let you buy a calling card from them and then you can even recharge it using a credit cared and the same site.
Different calling cards allow different amounts of time depending on how much money is on them and where you are planning on calling. While there are dollar phone cards out there they are good for quick calls to close countries or to places within the same country. Most calls will cost you at least 3 cents per minute so a dollar phone card is not going to get you very far.
My family and friends buy phone cars in denominations around $20. These are good if you are planning on making multiple calls. The dollar pone cards are best for quick emergency calls and nothing more. Even if the dollar phone cards are rechargeable it would be a waste as you would have to recharge after the first use. If you are planning on getting a rechargeable phone card you might as well start out with one with a decent amount of time on it.
All in all phone cards come in very handing if you are traveling abroad, and it is always a good idea to stock up on them in case you do not have enough cash to make a call from a pay phone.