We have a few Trio 8800 running release 5.7.2.3123. The phone does register and we can place and receive calls.
The Trio 8800 phones are in our conference room. I would like to configure them to display the room resource calendar so users can click on the meeting notice in the calendar to start there meeting.
The user in which the phone is registered to in O365 has the following licenses assigned.
The exchange server URL should point to https://outlook.office365.com/EWS/Exchange.asmx for Office 365 calendaring.