Calls can sometimes fail due to telephony-related issues. When this happens, an error message will be recorded in the call logs.
You can view the specific error by clicking into a failed call entry in your dashboard.
These error messages follow standard SIP/telephony response codes. To help you quickly identify the issue and resolve it without needing to contact support, we’ve provided simple explanations and recovery steps.
Error Message shown in your call logs.| Error Message | What It Means | How to Fix It |
|---|---|---|
| Telephony - 400 - Bad Request | The API request is malformed. | Check request format and parameters. |
| Telephony - 400 - Invalid phone number | The number format is not valid. Usually the TO header. | Correct the number (E.164 format recommended). |
| Telephony - 400 - The number +1234567 is unverified. Trial accounts may only make calls to verified numbers. | Twilio Trial accounts can only call verified numbers. | Connect your own Twilio account. |
| Telephony - 401 - Unauthorized | Invalid or missing authentication. | Re-check your API credentials or login session. |
| Telephony - 403 - Caller ID is unauthorized. A Valid Caller ID is either a DID you have purchased from Twilio or a verified Caller ID. | Your Caller ID is not verified. | Use a purchased DID or verify your Caller ID in your account. |
| Telephony - 403 - Forbidden | You don’t have permission to place this call. | Verify your account permissions or try a different number. |
| Telephony - 403 - Invalid credentials | Authentication details are incorrect. | Double-check your API key or login. |
| Telephony - 403 - No International Permission | Your account is not allowed to make international calls. | Enable international calling in your account settings or contact your carrier. |
| Telephony - 403 - Phone number is blacklisted | This number is blocked by the system. | Use a different phone number. |
| Telephony - 404 - INVALID DNIS | Invalid destination code. | Verify the number and retry. |
| Telephony - 404 - No VoiceURL | No URL configured for call instructions. | Add a valid VoiceURL in your account. |
| Telephony - 404 - Not Found | The number you dialed does not exist or cannot be reached. | Double-check the phone number format and try again. |
| Telephony - 404 - Service closed for this number | Calls to this number are not allowed. | Use another number. |
| Telephony - 408 - Request Timeout | The call setup took too long. | Retry later, or check your internet connection. |
| Telephony - 410 - Gone | The number is no longer in service. | Contact the recipient for an updated number. |
| Telephony - 480 - Temporarily not available | The recipient is not available. Happens on no answer after x seconds. | Try again later. |
| Telephony - 481 - Call leg/Transaction does not exist | The call reference is invalid or expired. | Start a new call session. |
| Telephony - 482 - Loop Detected | The call routing created a loop. | Investigate your SIP routing for loops. |
| Telephony - 483 - Too Many Hops | The call routing exceeded limits. | Investigate your SIP routing for loops. |
| Telephony - 484 - Address Incomplete | The dialed number is incomplete or invalid. | Check the full number has the including country/area code. |
| Telephony - 484 - Call Rejected | The recipient explicitly rejected the call. | Try later or contact the recipient. |
| Telephony - 486 - Busy Here | The recipient’s line is busy. | User is busy - Call back later. |
| Telephony - 487 - Request Terminated | The call was canceled before it connected. | Re-initiate the call if needed. |
| Telephony - 488 - Not Acceptable Here | The network can’t handle the call request. | Check your call media settings (codecs) and retry. |
| Telephony - 500 - Decline | Something went wrong on our end. | Retry. If persistent, contact support. |
| Telephony - 500 - Internal Server Error | Something went wrong on our end. | Retry. If persistent, contact support. |
| Telephony - 500 - Service Unavailable | The system or carrier is overloaded. | Retry. If persistent, contact support. |
| Telephony - 501 - Not Implemented | The feature isn’t supported. | Retry. If persistent, contact support. |
| Telephony - 502 - Bad Gateway | Network issue between systems. | Retry. If persistent, contact support. |
| Telephony - 503 - No Routeplan Found | The system could not find a valid route. | Retry. If persistent, contact support. |
| Telephony - 503 - Service Unavailable | The system or carrier is overloaded. | Retry. If persistent, contact support. |
| Telephony - 504 - Request Timeout | The call setup took too long. | Retry. If persistent, contact support. |
| Telephony - 603 - Decline | The call was rejected by the recipient’s network or device. | Retry. If persistent, contact support. |
| Telephony - 608 - Rejected | The call was rejected due to policy. | Retry. If persistent, contact support. |
| Telephony - 608 - Termination needs STI IDHDR | Call requires caller ID verification (STIR/SHAKEN). | Retry. If persistent, contact support. |