Hi Elane,
I assume that you will be using your US cell phone and service while abroad (in other words you will not replace your US sim card with a Belgium sim card). If so here’s a page explaining in detail how international roaming works.
To specifically answer your questions:
-to call the US you will dial 00 1 + area code + local number
-to call another US cell in Brussels use the same procedure above
-to call any Belgium numbers just dial these numbers the way locals dial them (no international codes here)
-to call Italy (landline or cell), use this page