[Effendi]

Wind with Dialoga International and you call the US at 0.125?/min directly with your phone. Otherwise there are surely cheaper solutions with callback services and so on.
You'll never get the perfect coverage, but compared to the "typical" US coverage you won't have problems in Italy with any of the 3 operators. Usually TIM is considered to have the most widespread coverage, followed by Voda and Wind, but it can change much from place to place. Since you won't be calling all the time and travelling so much I think Wind can be ok even if it has the weakest coverage compared to the other 2.