Calling different phones while in Italy
 

I'm going to be in Italy with an Italian SIM card in my phone (+39 xxx-xxxx-xxx). My friend, will be bringing her US phone, which will still contain her US SIM card. My question is how do I call her when we are both in Italy? Do I need to use the +01 USA country code even though she's not in the US? Or do I simply dial her 10 digit number (xxx-xxx-xxxx)?

Use the +01 code.

Does she know how much it will cost her using her phone?

It's +1, not +01. 001 also works, but make sure you don't use the +.

You always can dial the international form of a number (+1 xxx-xxx-xxxx). The call will be billed according to the correct national or roaming fees. When you dial a number without international prefix the system assumes you are looking for a phone in the country where you are. Say you dial yyy-yyy-yyyy in Italy: it is a call to +39 yyy-yyy-yyyy; you dial the same number in Germany; it is a call to +49 yyy-yyy-yyyy. And so on.

When you are memorizing phone numbers in your contact list, it is better to memorize them in the international form, that is always correct and works everywhere you bring your phone.

sorry, I knew you had to use the international code, not use the 10 digits that we use in US to call US number. Blame it on being late and too much wine. :o)

Thanks. I suspected that I would need to use the +1 code, but I wasn't certain. asps clarified the situation rather effectively for me. Once my friend gets some idea as the the costs of using her US phone in Italy, she may well change her mind.