Have you turned of the internet connection on your I phone?
Settings > General > Mobile Data > Mobile Data Network and then turn off Mobile Data.
Also if internet connection not disabled then usage can be for emails, Apps updates etc.
Surface Pro (2017) vs Surface Pro 4
20 groundbreaking 3D animation technologies coming to Siggraph 2017
iPad Pro 12.9 vs Surface Pro 5
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.