Error Code Directory

HTTP Retrieval Failure

Twilio could not reach your webhook URL when trying to make or receive a call.

SDP Negotiation Failed

The SDP offer/answer exchange failed during call setup, usually due to codec mismatches or network issues.

Dial: Cannot Route to Number

Twilio cannot route the outbound call to the dialed number — often caused by invalid formatting or unverified caller ID.

Dial: Invalid Method Attribute

The method attribute on your <Dial> TwiML verb is invalid — must be GET or POST.

Media Connection Failed

The media stream between Twilio and your endpoint could not be established, resulting in no audio.

Invalid TwiML Verb

TwiML document contains an invalid or misspelled verb element that Twilio does not recognize.

Invalid Attribute

A TwiML verb contains an attribute that is not valid for that verb type.

Invalid URL

A URL attribute in TwiML is malformed, empty, or points to an unreachable resource.

Dial: Attempt to Call Restricted Number

Attempt to dial a number on the account's blocked or restricted list.

Authenticate

Twilio rejected the request due to invalid credentials — account SID or auth token is incorrect.

Phone Number Already Verified

The phone number is already registered as a verified caller ID on this Twilio account.

Connection Error

The media connection failed — Twilio Client was unable to establish an audio stream for the call.

Call Recording Failed

Recording could not be started or completed for this call.

Unprocessable Entity

The request body is malformed or missing required fields — check your voice_id, model_id, and text parameters.

Rate Limit Exceeded

You've exceeded the ElevenLabs API rate limit. Back off and retry with exponential delay.

Unauthorized

Your ElevenLabs API key is invalid, expired, or missing from the request headers.

Service Unavailable

ElevenLabs servers are temporarily overloaded or under maintenance. Retry after a short delay.

Bad Request

Malformed request, missing required fields, or invalid parameter values sent to the ElevenLabs API.

Internal Server Error

ElevenLabs server error — likely transient, retry with exponential backoff.

Webhook Timeout

Vapi webhook delivery timed out — your server did not respond within the required time window.

TTS Generation Failure

Voice synthesis failed during an active call — the caller experienced silence where assistant speech was expected.