If you have a problem with Microsoft Teams not showing Calendar for users that have on-prem mailbox then you missed some of the steps in making a hybrid. To be honest, sometimes it is not very easy and clear what exactly you have to do.
In my setup I am running Exchange 2019 on Windows 2019 server.
1. First step is to run Microsoft Office Hybrid Configuration Wizard
This step is necessary even if you ran AD Connect and selected Exchange Hybrid. Go through the Wizard and when you finish, it will tell you you need to configure OATH manually as it is not part of the Wizard (in year 2020)
2. Follow all the steps on
For me step 8. was not necessary as I am running Exchange 2019.
After successful implementation of OATH connection Calendar will be immediately available to Teams users.
Test connection with:
Test-OAuthConnectivity -Service EWS -TargetUri https://outlook.office365.com/ews/exchange.asmx -Mailbox
-Verbose | Format-List
Exchange Online PowerShell
Test-OAuthConnectivity -Service EWS -TargetUri
/metadata/json/1 -Mailbox -Verbose | Format-List