Consider the following scenario in Microsoft Skype for Business Server 2015:
You create an RGS workflow that's configured to use a queue that's set to forward timed-out calls to an SIP address.
A PSTN call is initiated to the RGS.
The agent accepts the call shortly before the call times out in the queue.
In this situation, the agent completes the supervised transfer and joins the original call, together with the target of the blind transfer.
To fix this issue, install the August 2021 cumulative update 6.0.9319.619 for Skype for Business Server 2015, Response Group Service.