In a Microsoft Skype for Business Server 2019 environment, consider the following scenario:
You enable anonymous PSTN activation by running the Set-CsDialInConferencingConfiguration -AllowAnonymousPstnActivation $true cmdlet.
You schedule a private conference and set the access level as “Anyone”. Note that the meeting must be a scheduled conference. Because if you use the Meet Now feature to start a meeting, the Skype for Business user will join the conference first.
A PSTN user dials in to the conference and make sure that you are transferred to the conference without waiting for an authenticated user.
You may join the conference from a Skype for Business client and make sure that the PSTN caller is already in the conference.
In this situation, the PSTN user is placed in holding until a Skype for Business user joins the conference. It’s expected that the PSTN phone should be transferred to the conference without waiting for an authenticated user.
To fix this issue, install the July 2019 cumulative update 7.0.2046.123 for Skype for Business Server 2019, Conferencing Attendant.