Symptoms
When you try to add a second PSTN call to an existing PSTN call in Microsoft Skype for Business Server 2015, you receive a "403 Forbidden" error message that states "PSTN Toll Bypass disallowed." However, you should be able to place the second PSTN call because the call is made through the same gateway and doesn't try to bypass toll charges.
Resolution
To fix this issue, install the August 2021 cumulative update 6.0.9319.619 for Skype for Business Server 2015, Front End Server and Edge Server.
