Symptoms
A Survivable Branch Appliance (SBA) user can't escalate a PSTN call to a conference call if the SBA user is trying to add another PSTN user to the call and if Location-Based Routing (LBR) restrictions are enforced in Microsoft Skype for Business Server 2019.
Resolution
To fix this issue, install the December 2021 cumulative update 6 for Skype for Business Server 2019, Core Components.