Please let me add my 2 cents here as I had a customer request regarding Microsoft Teams calendar with Exchange on-premises. I was able to access the user's calendar, create edit delete or join a meeting through Microsoft Teams with the mailbox on Exchange on-premises and no Exchange Full or Minimal hybrid configuration.
Many articles or colleagues pointing to implement Exchange hybrid to get the calendar in Microsoft Teams to work with Exchange on-premises mailboxes.
This is not true, neither a requirement from Microsoft, it is just a "good" recommendation.
What here truly is needed are the following steps:
Azure AD Connect with Exchange Hybrid attributes sync
1.1 From that point, the Exchange Online is aware of the on-premises mailbox
Exchange on-premises 2016 CU3+
Exchange on-premises must be accessible from MS Teams IP
3.1 Autodiscover will be used from MS Teams backend services to discover and access the Exchange on-premises
3.2 Firewall rule to allow traffic may be required - MS Teams backend services IPs -
http://aka.ms/ipurlws3.2 Create Autodiscover DNS entry in your public DNS and point to your Exchange Server if it is not set
3.3 Exchange on-premises should answer to requests over 443 with a public certificate which covers autodiscover and the external hostname of the server
OAUTH has to be configured
4.1
https://docs.microsoft.com/en-us/exchange/configure-oauth-authentication-between-exchange-and-exchange-online-organizations-exchange-2013-help4.2 Step 1-5 should be enough, test if OAUTH is working with the steps at the bottom of the article.
Please message me if you face any issue and I would be happy to assist you
Spikar
