In Microsoft Skype for Business Server 2015, when you connect a public switched telephone network (PSTN) call to a user through a mediation pool by having REFER enabled, the call cannot be transferred to a federated user, and the call is dropped. The transfer (REFER) request to the federated endpoint fails and generates a "403 forbidden" error. On the remote federated side, the transfer (REFER) request is rejected by the Edge Server, and you receive the following error message:
ms-diagnostics: 1019;reason="Referred-By header parameters are not valid";ErrorType="The referrer authentication parameters are not recognized";Referrer="firstname.lastname@example.org";HRESULT="0xC3E93C52(SIPPROXY_E_REFERRER_IDENTITY_INVALID)";
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.