Lync 2013 or Lync 2010 can't connect to the Skype for Business Online (formerly Lync Online) service through a proxy that is configured by using a file://-based Proxy Auto-Configuration (PAC) file.
You can't connect to the Skype for Business Online service when you're using Lync 2013 or Lync 2010. When you view Network Monitor, traces show multiple TCP SYNC requests for login.microsoftonline.com.
To resolve this issue, use one of the following methods, as appropriate for your situation.
Method 1 (for end-users): Change the proxy settings in Internet Explorer
To resolve this issue, take one of the following actions in Internet Explorer:
Click to select the Use automatic configuration script option, and then configure an HTTP-based proxy PAC file.
Click to select the Automatically Detect Settings option, and then deploy Web Proxy Auto-Discovery (WPAD).
To do this, follow these steps:
Start Internet Explorer.
Click Tools, and then click Internet options.
Click the Connections tab, and then click LAN Settings.
In the Local Area Network (LAN) Settings dialog box, click to select either Automatically Detect Settings or Use automatic configuration script, depending on your network configuration.
For more information about how to configure an HTTP-based proxy PAC file, go to the following Microsoft website: