Very easy solution.
use the + sign.
To dial just press and hold 0, a plus sign will appear, then 1 (555) 555-1212 (not real number)
+15555551212
The plus sign takes care of all the complictions.
When in a country just dial the plus and the number as locals would dial it. The plus elimiates the access code (011) and the country code (39) the cell system does the work for you. And if you want to call your friend who also is in Italy with a US cell you jsut dial their same US number, just add +1 before the area code.
The great thing is that you can program this into your phone book as that person's number with the plus sign. When you get back to the US the system just ignores it. All your numbers still will dial.