Pushed the new outbound IVR flow to prod yesterday. Flow ID is fl-8829-xkq, running Architect v2023.11.0. Setup routes calls through the Ohio BYOC trunks using a standard SIP trunk object block. Registration sticks fine, but the moment the flow hits the Play Prompt block after the answer event, the carrier SBC throws a 488 Not Acceptable Here. Console shows the media path negotiating G.711ulaw, which matches the trunk config exactly. Toggled the DTMF relay to RFC 2833 just in case. Didn’t change a thing. The carrier engineer swears their SBC expects SDP re-offers on every prompt swap. Platform just sends one initial INVITE and holds the path open. Trunk utilization sitting at 12%, so it’s not a capacity bottleneck. Failover routing never triggers since the initial handshake dies before the call bridges. Rerouted the same flow to the Virginia group and the prompts play clean. Ohio keeps choking on the media setup. Logs show the 488 coming back on the ACK, right after the flow tries to inject the first audio chunk. Checked the outbound routing rules, all matching the correct region tag. Media config block has the usual fallback codecs enabled. Last packet capture shows the SDP answer dropping the a=rtpmap:0 PCMU/8000 line completely.