If you have Skype for Business installed on a computer that is not joined to the domain you may receive an error when attempting to sign in and use the application while on-campus. A certificate must be installed to solve the problem.
Locate the downloaded file and begin the installation by double-clicking the file. Click "Open" when prompted.
Click "Install Certificate" to continue the installation.
You can choose to install the certificate for either the current user or everyone on the computer and then click "Next".
Select the radio button next to "Place all certificates in the following store" and click "Browse".
Place the certificate in the "Trusted Root Certification Authorities", click "OK" and then "Next".
Click "Finish" to complete the certificate installation.
Restart Skype for Business and you should be able to sign and use the product.