Twilio error 31009 is a transport-layer failure indicating that the underlying WebSocket connection used for signaling or the RTP media transport channel was dropped while the SDK was active. This error can occur at initialization when the signaling WebSocket cannot be established, or mid-call when a previously stable connection breaks unexpectedly. Because 31009 affects the communication channel between the SDK and Twilio's infrastructure, it typically results in a call being disconnected or a registration being lost.
What Causes This Error
The most common cause is a proxy server or load balancer sitting between the browser and Twilio's signaling servers that terminates WebSocket connections after an idle timeout, closing the persistent WebSocket the SDK depends on for signaling even during active calls where no signaling messages are being exchanged at the transport level. NAT (Network Address Translation) timeout on routers and firewalls that drop UDP port mappings for RTP media streams that have been idle for more than 30 to 60 seconds, which some carriers and enterprise firewalls enforce, causes the media channel to close mid-call and generates a 31009 transport error. Browser tab switching or device sleep states on mobile browsers (particularly iOS Safari, which aggressively suspends background tabs and JavaScript execution) interrupt the WebSocket keepalive heartbeat and cause the connection to be closed by the server, resulting in 31009 when the suspended tab becomes active again. Network interface changes such as switching from WiFi to cellular data mid-call cause the client's IP address to change, which invalidates the existing ICE candidate pair and media channel, producing a 31009 if the SDK does not successfully execute an ICE restart.
How to Fix It Step by Step
Enable WebSocket keepalive pings in your SDK configuration by passing the closeProtection option when initializing the Device, which instructs the SDK to send periodic WebSocket ping frames to prevent proxy idle timeouts from closing the connection. Handle the Device.on('error') event for code 31009 by calling Device.register() to re-establish the signaling connection and Call.reconnect() on any active call to attempt an ICE restart, which is the standard recovery sequence for transport failures. Check whether your infrastructure has any WebSocket connection timeouts configured: for Nginx reverse proxies, set proxy_read_timeout to at least 3600 seconds; for AWS ALB, set the idle timeout to 3600 seconds in the target group settings; for Cloudflare, ensure WebSocket timeout is not configured below the default. Test the specific scenario of long-duration call WebSocket keepalive by holding a test call open for 5 minutes without speaking and monitoring the browser console for any WebSocket close events, which confirms whether the proxy timeout is the cause.
How to Prevent It from Recurring
Implement the Device's built-in reconnection handling by subscribing to the Device.on('unregistered') event and automatically re-registering with a fresh access token, combined with an exponential backoff strategy that retries registration up to 5 times before notifying the user that connectivity has been lost. For mobile web applications, use the Page Visibility API to detect when the tab becomes hidden (document.addEventListener('visibilitychange', handler)) and proactively handle the SDK state when the page returns to visibility, as iOS Safari's background suspension typically causes a 31009 that needs immediate reconnection handling. Configure your TURN server session timeout to be longer than the expected maximum call duration plus a safety buffer: fetch NTS token credentials with a time_to_live value of at least 7200 seconds for calls that may last up to 2 hours. Add call quality monitoring using the Connection.on('warning') event, which fires when packet loss or jitter exceed thresholds, giving you early warning of network degradation before it escalates to a full 31009 transport failure.
When to Call a Specialist
If 31009 transport errors are occurring consistently at a predictable interval (for example, always after exactly 30 or 60 seconds), this strongly indicates a network infrastructure timeout and a specialist can identify the specific device in your network path that is enforcing the timeout and recommend the correct configuration change. For mobile browser 31009 errors caused by iOS Safari's background tab suspension, a specialist can implement a Web Audio API based audio keep-alive technique that prevents iOS from suspending the JavaScript thread during calls, which is a non-obvious workaround that requires precise implementation. You should also escalate if 31009 errors are correlated with specific geographic regions or ISPs in your user base, as this pattern indicates a carrier or ISP-level NAT timeout behavior that requires either TURN relay configuration tuning or application-level reconnection logic tailored to that network environment.
Conclusion
Error 31009 is a transport layer disconnection that is resolved by configuring WebSocket keepalive, handling reconnection in the SDK error handler, and eliminating proxy timeout configurations that close long-lived connections. If this error is blocking your production system, contact our team and we will diagnose and fix it within the hour.
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