Are iPhones always using internet data?

Two of my room mates have an iPhone that are connected to our Wi-Fi. Even when they're not browsing the internet with their phones and their phones are just sitting there, are they still using data from the internet? If we go over our internet data for the month we have to pay extra for every gigabyte that we use, so I'd like to know if their phones are taking internet data.

Yes there's a chance there iPhones are using data. There are always things running on an iPhone. Let's say for Whatsapp, the iPhone needs to search for incoming messages every now and then so that uses data. Although that doesn't use a lot of data, it still uses data. If they are downloading an app but lock there phone, there phone is still downloading the app and using data.