Assume that you are Location-Based Routing (LBR) enabled on a LBR enabled network site in Microsoft Skype for Business Server 2015. Then, you start a public switched telephone network (PSTN) call. When you right-click and then select the Skype Call option to add a non-LBR enabled user from a Skype for Business mobile client, the user is added to the PSTN call successfully. After you apply this update, the call will be failed because the call is from an LBR enabled gateway to a user at an unknown location.
To fix this issue, install the January 2018 cumulative update 6.0.9319.514 for Skype for Business Server 2015, Front End Server and Edge Server.