The outbound flow breaks at the INVITE stage. First, the setup loads Embeddable Client App SDK v4.2.1. Next, callSession.makeCall() triggers the request. Network tab shows the SIP INVITE hitting the Genesys edge, then the server drops it with SIP/2.0 488 Not Acceptable Here. Payload carries the correct Supported: replaces header, but it’s getting rejected. Standard browser client works instantly. Custom desktop won’t accept the media negotiation. Audio tracks stay dead. Logs point to a codec mismatch in the SDP answer block.