Consider the following scenario:
You access the Internet through a proxy server that requires authentication.
The proxy server is configured for a language that uses extended ASCII characters.
You try to join a Skype meeting that's sent by a non-federated partner in Microsoft Skype for Business 2016.
In this situation, you can't join the Skype meeting successfully.
This issue occurs because the response from the proxy server contains ASCII characters greater than 127.
To fix this issue, install the March 6, 2018, update (KB4011725) for Skype for Business 2016.