Demystifying Hybrid Free/Busy: what are the moving parts?

Mirela_Buru 

Here is an update about on my case "Free/Busy" for on-premise Exchange 2016 was not functioning as expected between two difference sites. 

The solution was related to TLS configurations. As we adopted the TLS 1.2 and the third-party tool was used to enable it. However, it was not enabled correctly, the value of Registry key "Enabled" was not (1) which cause the problem!

Correct the value on Client/Server TLS 1.2 to (Enabled (1)) on all Exchange servers within the network solved the problem. 

Here are some links related to TLS.

Exchange Server TLS guidance, part 1: Getting Ready for TLS 1.2

https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-1-getting-ready-for-tls-1-2/ba-p/607649

Exchange Server TLS guidance Part 2: Enabling TLS 1.2 and Identifying Clients Not Using It

https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-2-enabling-tls-1-2-and/ba-p/607761

Exchange Server TLS guidance Part 3: Turning Off TLS 1.0/1.1

https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-server-tls-guidance-part-3-turning-off-tls-1-0-1-1/ba-p/607898

Hope this will be helpful.

Best Regards, 

Yaseen