You're hosted on a LBR (Location-Based Routing) enabled pool of Microsoft Skype for Business Server 2015. When you initial a conference call, if one of the participants joins the call by using the "Call me at" feature for his local meeting room device such as Polycom Trio, the later participants can't join the conference by using a Skype for Business client. However, they can dial in by using PSTN to join the meeting.
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.
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