Consider the following scenario in Microsoft Skype for Business Server 2015:
You set up forwarding on Shared Line Appearance (SLA) group Missed Call Option to User C if SLA is busy by using the Set-CsSlaConfiguration cmdlet.
User A calls User B.
User B forwards the call to SLA.
In this situation, the forwarding from SLA to User C fails.
This issue occurs because the Invite sent when call is forwarded by SLA contains both Referred-By headers.
To fix this issue, install the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Front End Server and Edge Server.