If you’re unfamiliar, the Office 365 connectivity test tool at https://connectivity.office.com can test your internet connection between an end users PC and the Office 365 service.
This is useful if the user is experiencing issues with call quality, or is having issues accessing Exchange, Onedrive or Sharepoint.
I recently encountered an issue with the tool where, upon finishing the detailed test (all 705 of them!), not all of the results were not displayed within the Details tab of the page.
To resolve this issue, we needed to ensure TCP 443 was open to connectivity.service.signalr.net on both the local users PC firewall, and the proxy server.
Once allowed, we re-ran the test and resolved the issue.