How to add other device to my iPhone
I would like to add other number on find my iPhone
iPhone 6s, iOS 11.2.1
You can't simply "add" another number (or device) to Find My iPhone. You must actually set-up Find My iPhone on the (each) device itself:
Set up Find My iPhone on all of your devices - Apple Support
Then you can use iCloud to find any/all of your devices: