In Microsoft Skype for Business Server 2015, a user creates a Skype meeting by using the Skype for Business for Mac or Skype for Business for iOS client. Then, some users dial into the meeting. However, the PSTN callers aren’t shown in the meeting participants list. It’s expected that all the meeting participants should be displayed.
To fix this issue, install the May 2020 cumulative update 6.0.9319.580 for Skype for Business Server 2015, Web Components Server.