Skype for Business for Mac

Hello,

i have an on-premise Skype For Business infrastructure 2015. While all users works perfectly, i have a user with Mac OS that has a problem.

When receiving an audio/video call works fine, when try to make an audio/video call from his Mac client call dropped.

The same user on Windows client works both (receiving/ make call).

The current client version is 16.24.

The latest is 16.26 (will be installed in a while).

Instant messaging is working fine.

Before install network monitor to capture the network traffic, could you please share an idea when and how to identify where is the problem?

Thank you very much..

Gregory