Working with a client that had deployed a Polycom Trio 8800, the phone was registered correctly however the calendar won’t load.
I looked at the Trio Status page and shows that EWS is deployed but the AutoDiscover EWS External URL is blank.
What I found interesting is it showed the: Configured Exchange URL as https://, so I checked the configuration Settings -> Applications-> Exchange Applications
The AutoDiscover was enabled, but the Exchange Server URL field was filled in however it should not use this field but it appears that it actually does.
In order to fix the issue, I had to disable the AutoDiscover field and remove the https:// from the Exchange Server URL and the enble the AutoDiscover field and Save the configuration.
Once the configuration was saved we checked the log file and the status page and it now shows the EWS URL and Calendar is enabled.