No, the plus plan will only allow roaming or calling from Canada and Mexico. Some have mentioned to make sure Wi-Fi calling is working and switch to it before leaving the US but you would only be able to use it while connected to someone's Wi-Fi, for this I don't think you would need to change plans. You might want to check into getting a temporary SIM while in Italy and use that if Wi-Fi doesn't work for you.
There's no need to upgrade to Visible+; just make sure to setup Wi-Fi Calling feature on your phone and test it (Plane Mode and have Wi-Fi connection) before leaving the U.S. of A.
Fai un buon viaggio!
I am going to Central America in a few weeks with the standard Visible service and WIFI calling enabled. I also have WhatsApp. Do I understand correctly that I can send and receive calls via WIFI (from Hotels etc.) like normal with no fees?