Twilio error 31480 directly maps to SIP response code 480 Temporarily Unavailable, which is returned by a SIP User Agent (UA) or PBX when it recognizes the target extension or user but cannot accept the call at the current moment. The 480 is explicitly a temporary condition, unlike a 486 (Busy Here) or 603 (Decline), and callers are expected to retry after a delay. Understanding what causes the 480 response from the destination SIP device is key to implementing the right handling strategy.
What Causes This Error
A SIP softphone or hard phone that is not registered with the SIP registrar at the time of the call is a primary cause: when the device is powered off, out of network, or has let its SIP registration expire, the registrar returns a 480 on its behalf to indicate the endpoint is temporarily unavailable. Do Not Disturb (DND) mode configured on a SIP phone or a SIP-enabled application causes the UA to respond with 480 for any incoming call while DND is active, which is intentional from the device perspective but appears as an error from the caller's perspective. SIP PBX systems configured with availability schedules (time-based routing rules) that route calls to a 480 response outside of business hours use this code to indicate the agent or queue is not currently accepting calls. Twilio SIP Domains configured with a SIP URI destination where the destination SIP server is temporarily overloaded or in maintenance mode may send a 480 back to Twilio as a load-shedding response.
How to Fix It Step by Step
Check the SIP registration status of the destination device by logging into the PBX or SIP registrar admin interface and looking up the registration record for the target extension: a missing or expired registration record confirms the 480 is caused by an unregistered device. If the 480 is DND-related, investigate whether the DND was set intentionally (in which case 480 handling in TwiML is the correct approach) or accidentally (in which case the user needs to deactivate DND on their device). In your TwiML, add an action attribute to the Dial verb pointing to a URL that handles post-dial outcomes, and in that handler check the DialCallStatus parameter for failed and the DialSipResponseCode parameter for 480, then implement the appropriate fallback (leave a voicemail via Twilio, play a message, or redirect to another agent). For Twilio SIP Domain destinations returning persistent 480 responses, check the destination SIP server's health and logs to confirm whether the server is overloaded or in maintenance, and if so implement TwiML-level failover routing to an alternative destination.
How to Prevent It from Recurring
Implement SIP registration health monitoring on your PBX that alerts your team when agent extensions go unregistered for more than 2 minutes, which catches devices that have gone offline before calls begin failing with 480. In your TwiML application, use the Dial verb's fallbackUrl or action handler to gracefully handle 480 responses by offering callers a voicemail option or routing them to a backup queue rather than playing a generic error message. Configure your SIP PBX with a registration expiry of 60 to 120 seconds and ensure all SIP phones and softphones are configured to re-register at half the expiry interval (30 to 60 seconds), minimizing the window during which a device can be temporarily unregistered. For time-based routing scenarios where 480 is expected outside business hours, implement explicit TwiML routing logic in your application that checks the current time and routes to the appropriate after-hours treatment without making a SIP call that will return 480.
When to Call a Specialist
If SIP 480 responses are occurring inconsistently on registered phones (the phone shows as registered in the PBX but calls still return 480), the PBX may have a registration synchronization issue between its internal state and the registrar database, which requires PBX-level debugging by someone familiar with the specific PBX platform. A specialist can correlate SIP packet capture data from the PBX with Twilio's Debugger logs to identify the exact SIP message exchange that produces the 480 and determine whether the issue is in Twilio's routing, the PBX configuration, or the SIP device itself. You should also escalate if 480 errors are causing a noticeable increase in abandoned calls or missed customer contacts, as the business impact warrants immediate investigation and a more robust failover architecture.
Conclusion
Error 31480 is a SIP 480 Temporarily Unavailable response from the destination endpoint that requires both a technical fix on the registrar or device side and a TwiML-level fallback strategy to handle unavailable endpoints gracefully. 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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