In your other thread, you say your US provider does not offer service overseas ("international roaming" that is).
Is your phone locked - do you know? That's important to know if you hope to buy a SIM of any sort for the phone and use it in Italy.
Also, what kind of phone is it exactly? Make/model? If it's a newer smart phone, it should work (if unlocked) with a mobile provider in Italy, if you buy a SIM. It will work on WiFI no mater what, though, for free, without any mobile service or SIM.
I find the most helpful benefit of my phone when traveling is for walk-around/public transit navigation. You need mobile service (a SIM card with data) for that. There are some navigation apps that let you download maps offline (while on WiFi), but they aren't as useful as navigating in real time with Google Maps. Last year for example, I was out taking pictures late in Venice. I had about a 30 minute walk back to my hotel...or I could take a vaporetto boat back. But how would I know at this hour which vaporetto to take, when it would leave, from where? I just brought up Google Maps and told it to guide me back to my hotel. It gave me walking directions to a vaporetto stop nearby, where I soon caught a boat back to my hotel. In the past, I would have had to browse the vaporetto schedules, find the stops...or just give up and wind up walking back instead of wasting the time figuring it out. Google Maps navigation has saved me an enormous amount of time with navigation like this vs. fooling with paper maps and bus/train/boat schedules. That's why I find having a SIM and mobile data (not just "free WiFi all over the place") so essential.
In Italy, if you will take trains, I found the Trenit app very useful and helpful. it lets you buy tickets on your phone. It also lets you figure out train schedules, real-time delays, which platforms trains arrive and depart from, etc. Again, helps to have mobile data with a SIM to check that stuff while you are on the train coming up to the next station where you get off and change.
For calling...besides WhatsApp, I have found Google Hangouts extremely useful for making free calls home to the US, even to landlines (only to US phone numbers; calls to non-US numbers cost a few cents per minute). Nice to be able to call your bank or airline and be on hold and not worry about paying a per-minute call fee. Free on WiFi or with mobile data (which may not be free).