How can I enable / turn on Lost Mode on the iPhone?
Lost Mode prevents a missing or stolen iPhone from making noises and locks the device down. If Find My iPhone is already enabled on the target device, it can be tracked with Lost Mode.
Follow these steps to put an iPhone into Lost Mode:
1. Open iCloud (www.icloud.com) from a web browser.
2. Sign in using your Apple ID.
3. Click Find My iPhone.
4. Select the All Devices menu at the top of the browser window.
5. Choose the device to be put into Lost Mode.
6. Click the padlock radar button marked Lost Mode (pictured above).
7. Follow the on-screen instructions.
The iPhone will be tracked, locked down, and the custom message and phone number you entered will be displayed on the device's screen until Lost Mode is cancelled.