In a Microsoft Skype for Business Server 2015 environment, you use a Unified Communications Managed API (UCMA) application to schedule an audio or video conference and dial out a PSTN number from the conference by using the AudioVideoMcuSession.BeginDialOut method. After the UCMA application performs the dial-out operation from the Audio/Video Multipoint Control Unit (AVMCU), the invitee accepts the call and joins the conference successfully. However, the conference organizer or other participants could not immediately see the invitee through the UCMA application. It may delay for four to five seconds. It's expected that the invitee can be visible in less than one to two seconds.
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015, Core Components.