| Telephony - 400 - Bad Request | The request is malformed. | Check the request format and parameters. |
| Telephony - 400 - Invalid phone number | The number format is not valid, usually in the To header. | Correct the number. Use E.164 format. |
| 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 | Authentication is invalid or missing. | Check the SIP credentials on the custom number, or your Twilio credentials. |
| 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 number you purchased, or verify the caller ID in your account. |
| Telephony - 403 - Forbidden | You do not have permission to place this call. | Check your account permissions, or try a different number. |
| Telephony - 403 - Invalid credentials | The authentication details are incorrect. | Check your SIP or Twilio credentials. |
| Telephony - 403 - No International Permission | Your account cannot call this country. | Enable calls to that country with your carrier. In Twilio, this is set in Voice Geographic Permissions. |
| Telephony - 403 - Phone number is blacklisted | The number is blocked. | Use a different number. |
| Telephony - 404 - INVALID DNIS | The destination number is invalid. | Check the number and retry. |
| Telephony - 404 - No VoiceURL | The number has no URL for call instructions. | Add a voice URL to the number in your Twilio account. |
| Telephony - 404 - Not Found | The number does not exist or cannot be reached. | Check the number format and retry. |
| Telephony - 404 - Service closed for this number | Calls to this number are not allowed. | Use a different number. |
| Telephony - 408 - Request Timeout | Call setup took too long. | Retry later, and check your network connection. |
| Telephony - 410 - Gone | The number is no longer in service. | Get an updated number from the recipient. |
| Telephony - 480 - Temporarily not available | The recipient did not answer before the ring timeout. | Try again later. |
| Telephony - 481 - Call leg/Transaction does not exist | The call reference is invalid or expired. | Start a new call. |
| Telephony - 482 - Loop Detected | The call routing created a loop. | Check your SIP routing for loops. |
| Telephony - 483 - Too Many Hops | The call passed through too many hops. | Check your SIP routing for loops. |
| Telephony - 484 - Address Incomplete | The dialed number is incomplete. | Include the full number with country and area code. |
| Telephony - 484 - Call Rejected | The recipient rejected the call. | Try later, or contact the recipient. |
| Telephony - 486 - Busy Here | The recipient’s line is busy. | Call back later. |
| Telephony - 487 - Request Terminated | The call was canceled before it connected. | Start the call again if needed. |
| Telephony - 488 - Not Acceptable Here | The network cannot handle the call’s media. | Check your codec settings and retry. |
