I've been meaning to write this up for a while. I mostly wrote it to scratch an itch for myself, but hopefully it's useful to some of you.
There are times when you may want to reduce the amount of data used on your cell phone. This could be because you are on a low-data plan or have simply used up the amount of data you have available for the month. iPhones have a large number of settings that affect data use, and I've tried to provide some useful guidance below. Please don't ask me for equivalent help on Android - I don't have the background to write this article for Android phones.
1) When at home, turn cellular data off in Settings/Cellular. This may seem obvious, but a lot of people leave cellular data turned on when at home and rely on also having a wifi connection to reduce their cellular data use. That works - mostly. The problem is that your iPhone wakes up from sleep periodically to perform background refreshes for apps, like fetching any new emails. In some cases, the wifi network can take a second or so to connect when waking up from sleep, while the cellular connection is already awake, and if that happens your phone may use cellular data. This isn't going to happen all the time, but my experience has been that if you keep cellular data on at home, it will be used to some extent.
2) Turn off iCloud backups over cellular data, , in Settings/iCloud/iCloud Backup. This prevents your phone from using cellular data to perform backups if it is plugged into a charger but does not have a wifi connection. The text on this screen says "This may cause you to exceed your cellular data plan". There's really no reason to do backups over cellular unless your phone rarely gets to back up over wifi. Missing a day or two of wifi backups is not something you need to worry about, so keep this setting disabled.
3) Disable WiFi Assist in Settings/Cellular (near the bottom). This feature allows the phone to use cellular data when WiFi connectivity is poor. I'd recommend keeping this turned off; if you ever do find yourself with poor WiFi, you can always turn off Wifi And turn on cellular data yourself
4) Control which apps can access cellular data in Settings/Cellular. This screen will show you a long list of your installed apps, and while you probably will want most of your apps to use cellular data, it's worth reviewing. You'll also see how much data each app has used on this screen, which can be very helpful in finding out exactly where your data usage happened. If nothing else, I'd recommend disabling all video apps (Nefllx, Youtube, TV, etc) since that will protect you against accidentally watching Netflix without turning on Wifi first. Video apps are very data hungry and unless you are being careful and intentional in their use, it's best to prevent them from using cellular data.
5) Control which apps can perform background refreshes in Settiings/General/Background Refresh. This one is more subtle - it controls whether an app can wake up in the background to check for updates. If the app is allowed to use cellular data, this can increase the amount of data it uses. In many cases, like email, this is reasonable - you want your phone to let you know when you get a new email. You probably have some apps, though, that really don't need to be updating themselves all day long, so this section lets you reduce the data used by an app without turning it off completely.
Monitoring Data Usage
Finally, you can't know if you have a problem with some apps (see #4) unless you have a good monitoring strategy. Unfortunately, the iPhone does not have a built in usage monitor other than the Settings/Cellular screen. That screen does not automatically "reset" usage when your plan renews each month, but you can tap "Reset Statistics" at the bottom on the day your plan resets. It's worth setting up a reminder to do it, though, since this is the only source of information on which apps are using your data. The Public Mobile My Account page will automatically keep track of how much data you've used this bill cycle, but it cannot break it down by app.
