Keep noted that Windows Phone 8.1 will reach EOL on the 2017-07-11 and will therefore no longer be supported by Microsoft.
That means you do not get any kind of support or updates. Users which still use Windows Phone 8.1 should upgrade to Windows Phone 10 if possible as explained here. Keep also noted that Mobile-Device-Management (MDM) Vendors might drop the Windows Phone 8.1 support soon.
