> For a complete page index, fetch https://docs.synthflow.ai/llms.txt. For full documentation content, fetch https://docs.synthflow.ai/llms-full.txt. # Telephony Errors > Telephony error messages and how to resolve them 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. --- ## How to Use This Table * Find the `Error Message` shown in your call logs. * Look it up in the table below. * Follow the suggested steps under **How to Fix It** to recover. --- ## Telephony Error Codes – Quick Reference | 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. | > Telephony error messages and how to resolve them