Get-MobileDeviceStatistics -mailbox XXXX |fl devicefriendlyname, devicemodel, deviceos, deviceimei
Deviceimei does not appear for all devices can you make all device has Deviceimei available and consistent and better
further investigating the issue by Microsoft O365 Team able to find out that the Outlook app does not have the required permissions to read some of the information from the phone (e.g. the device IMEI). The connections made with Outlook for mobile will not provide that information.
Also IMEI does not appear for devices that do not have data connectivity (e.g. tablets).
Tomas Cabuk commented
This functionality is very usable for the corporate environment, where you need the device identification for reporting. Without the clear device identification, you are not able to create any management usage overviews etc ...