I think CarnivalPride is asking about calling cards, not SIM cards.
If so, there are scratch cards sold at tobacco shops and newsstands for denominations of 5 euros up. There are several brands, and, as far as I know, they're all pretty much alike. The most important thing is to get one that's specifically for North America, if that's where you'll be calling.
The cards have various numbers you can call to initiate the call process. There usually will be a toll-free number and a local number that you can call from landline phones, such as your hotel phone; you get better rates with the local number, but then you have to pay for the local call as well. I would use the toll free number from a landline phone. Then they will have numbers to use if you're calling from a cell phone or a pay phone.
I used to use the Sisal Edicard, which has a specialty called Europa & USA. This is not a scratch card; it's a little receipt that's printed on the spot, wherever SuperEnaLotto tickets are sold.
There will be prompts in various languages, depending on the region you've chosen. The ones I've used didn't need the international prefix, just the country code and number. In case you need it, the international prefix is "00" and the country code for the US is "1".
The rates are the same all day long, as least on the cards I've used, and they can be used all over Italy.
The scratch cards Lynn is talking about are for topping up your talk time for cell phones. Everything she said about them is true, except that they don't usually sell them in phone stores; if you want to top off your minutes in a phone store, they have a special terminal to do do. You usually can buy the scratch cards for cell phones in the same places you can buy the calling cards.
The phone cards that work in public phones have very high rates for overseas calls. These are not scratch cards; they are little plastic cards where you break off one corner before inserting it in the slot in the phone. They are not good value, but no calls from public phones are good value. Even if you use a scratch calling card, you'll get a much better deal calling from your hotel than from a public phone.