Since notifications pushed to your device are carried over cellular data, you may see a small amount of cellular data usage even while connected to Wi-Fi.
If you tap a notification on your connected Apple Watch, any subsequent data usage may be carried over the VZW cellular network.
The steps below apply to the following models:
iPhone X / XS / XS Max / XR
iPhone 8 / 8 Plus
iPhone 7 / 7 Plus
iPhone 6 / 6 Plus / 6s / 6s Plus
iPhone 5 / 5c / 5s
iPhone 4/ 4s
Where available, touch the Status bar (at the top) then drag to the bottom.
The image belows is an exmaple.
To view more notifications (e.g., Weather, Stocks, Reminders, Calendar, Mail, Messages, etc.), swipe the screen left.
For iPhone 4 / 4s, tap Notifications, then tap the desired notification.
Notifications provide alerts and related info for iOS apps (e.g., Sounds, Alerts, Banners, Badges, etc.).
For additional info, refer to iOS: Understanding Notifications.