Consider the following scenario:
You're a Location-Based Routing (LBR) enabled user and located on a LBR enabled site in Microsoft Skype for Business Server 2015.
You successfully start a peer-to-peer call with a public switched telephone network (PSTN) user by using the Skype for Business on Mac client.
Then you escalate the peer-to-peer call to a conference by inviting another Voice over Internet Protocol (VoIP) user who is located on the same site as you.
In this scenario, you may notice that the user can't be reached because of conferencing service issues. It's expected that the user can join the conference without any problem as the escalation happens to the same site's user only.
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015, Front End Server and Edge Server.