Lync 2011 not working

Does Lync Mac 2011 work?

With OS10.11.6? (or a particular OS?)

Is there a secret to making this work?

I have a user who said it worked previously, now it doesn't work on their MacBook.

Another IT person said they heard that a Mac system update might have broken Lync Mac.

I tried installing it on an extra MacBook Air that we have.

When I try to Sign In, I get a general error message;

"Sign in to Microsoft Lync failed because the service is not available..."

I have tried Advanced, and entering the Internal and External server name and get the same error.

Does anyone know if there are any upgrades or tricks to making this work?

Thank you,

Bill