When you dial 0 from a Microsoft Lync Mobile Client dial pad, the call is dropped immediately. This problem occurs when the following conditions are true:
You have configured 0 (zero) as the operator’s hotline number on your Lync Mobile Client device (Windows Phone, Android, or iOS).
You dial 0 on the dial pad and then try to complete the call.
The Lync Server UCWA component treats 0 as an invalid number. Therefore, a "Bad Request" response is triggered, and the call is terminated.
To resolve this issue, install the May 2015 Cumulative Update 5.0.8308.887 for Lync Server 2013 Unified Communications Managed API 4.0 Runtime.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.