Assume that you are Location-Based Routing (LBR) enabled on an LBR enabled network site in Microsoft Skype for Business Server 2015. Then, you start a call together with a public switched telephone network (PSTN) user. When you try to transfer the PSTN call to another user who is only Enterprise Voice (EV)-enabled and located at a non-LBR-enabled network site, the call is routed to his voice mail if the non-LBR user isn't available. After you apply this update, the call transfer will fail because the call is from an LBR-enabled gateway to a user who is located at a different location.
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Front End Server and Edge Server.