[iMore Question]

even if am not using my phone

[Rob Phillips]

Welcome to iMore! iPhones do a lot of things in the background that use data but you can take steps to minimize these processes.
Go to Settings > Cellular. From there you can toggle off apps that you don't want using cellular data. While you're there I recommend scrolling all of the way down and toggling off Wi-Fi Assist.
Go to Settings > Mail, Contacts, Calendars. If you're okay not getting immediate notification of incoming mail you can turn off push notifications and change them to fetch. You can also change the fetch interval. The greater the time interval, the less data it will use. You can optionally change it to manual, so your iPhone will only check for new mail when you launch your Mail app.

[Just_Me_D]

Alternatively, go to Settings → General → Background App Refresh, and then disable the items that you think don't need to have the option enabled.

[robertk328]

Go to Settings > Cellular. From there you can toggle off apps that you don't want using cellular data. While you're there I recommend scrolling all of the way down and toggling off Wi-Fi Assist.

Settings -> Cellular will also show you what apps are using the most data so you can choose to disable them or not. If you disable they will only pull data while on wifi so beware if you have a wifi cap as well. The amount shown is cumulative so you should reset it if you want to monitor at certain times. We reset each month on the account date so we can keep track of data usage.