> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cognigy.com/llms.txt
> Use this file to discover all available pages before exploring further.
# SIP Response Codes
> SIP response codes cover statuses from 200 to 603, with meanings and troubleshooting steps for voice AI Agent issues.
This page provides guidance on SIP response codes, their meanings, and potential actions to resolve issues, supporting you in troubleshooting voice agents.
For troubleshooting, check the [Alerts](/voice-gateway/webapp/alerts) page in the Self-Service Portal for any received alerts.
## SIP Call Response Codes
The following codes are related to call signaling, for example, when Voice Gateway establishes, transfers, or ends calls.
You can find these codes in Cognigy.AI [Logs](/ai/agents/test/logs) of your Agent under `sip_status`, or in downloaded [PCAPs](/voice-gateway/webapp/recent-calls#call-details) for specific calls.
| Code | Error | Description | What to Do? |
| ---- | ----------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| 200 | OK | The call was successfully answered. | No specific actions are required. |
| 202 | OK | The request has been accepted for processing, but processing is not yet complete. | No specific actions are required. |
| 400 | Permission Denied | The server refuses to authorize the request for various reasons, including the user not having the necessary permissions or using incorrect credentials. | - Check the request credentials (for example, user name and password) and permissions.
- Ensure the request is being sent to the correct server and endpoint.
- If there are a large number of requests with status 403 from the same IP, consider blocking the IP. |
| 403 | Invalid Caller ID | The incoming call was rejected by the carrier. | - Check the PCAP for detailed information on the cause of the failure.
- Some SBCs like Twilio may reject calls from unknown numbers. In this case, it might be necessary to configure the P-Asserted-Identity header for the Transfer. |
| 404 | Not Found | A call to a phone number not set up in the Voice Gateway Self-Service Portal may indicate a SIP spam attempt. | - Check the Carrier or Phone numbers configurations in the Self-Service-Portal.
- Verify the request URI provided in your Transfer Node is correct. The issue may be due to a mistyped address or an outdated record.
- Investigate further for troubleshooting and possible IP blocking. |
| 408 | Request Timeout | The SIP endpoint was unresponsive, and the server did not receive a complete request message within the time it was prepared to wait. | - This often indicates network issues or a server problem. Try sending the request again after some time, or investigate network connectivity issues at a lower level.
- Check and compare Voice Gateway PCAP for a specific call with your provider's PCAP for that call. |
| 480 | Temporarily Unavailable | There is no [application](/voice-gateway/webapp/applications) configured for an incoming call, or if there are no more resources available to handle a call (the system is at maximum capacity). | - Check whether an `application` is properly associated with the Phone number in the Self-Service Portal.
- Retry after some time. If the issue persists, contact the callee by other means to confirm their availability. |
| 487 | Request Terminated | The callee either rejects the call or fails to accept it within the defined timeout. Within the Flow, you will receive the [NO\_ANSWER](/voice-gateway/references/events/NO_ANSWER) call event. | No specific action is needed if the termination was intentional. If unexpected, inspect the call Flow for issues, for example, faulty Code Nodes. |
| 488 | Not Acceptable Here | The incoming call requested a codec or a feature in the Session Description Protocol (SDP) that the Voice Gateway does not support, or it provided an unacceptable parameter. | Check the codecs or any other session parameters being used and ensure they are supported by both the caller and callee. |
| 500 | Server Internal Error | The application error. The server encountered an unexpected condition that prevented it from fulfilling the request. This is a server-side issue. | Attempt the request later or contact the server administrator if the problem persists. For example, if the Voice Gateway responded with this code, contact Customer Support. If your carrier responded with this code, contact your provider. |
| 503 | Service Unavailable | Voice Gateway is unable to process the call due to specific reasons, including an `inactive` account status or reaching the `maximum call limit`. | Contact Customer Support or your Customer Success Manager at Cognigy. |
| 603 | Decline | The callee's end system was contacted successfully, but the user explicitly rejected the call, or there was an issue connecting to the Voice Gateway Endpoint. | - If it was a normal rejection of a call, no further action can be taken to establish the call. If necessary, try contacting the user through alternative means.
- If this response was not expected, check the Self-Service Portal configurations of your Carrier as well as the Alerts page.
- Check Cognigy AI logs for any error log messages that could indicate the root cause. |
## SIP REGISTER Response Codes
The following codes are related to the SIP registration of a Voice Gateway carrier with a SIP trunk provider.
You can view the result of recent registration attempts on the **Edit carrier** page in the Voice Gateway Self-Service Portal. For more information, see [Register Status](/voice-gateway/webapp/carriers#register-status).
The `401 Unauthorized` and `407 Proxy Authentication Required` responses are part of the standard SIP digest authentication handshake and aren't errors on the first attempt. Treat them as an issue only if the registration still fails after the request is resent with credentials.
| Code | Error | Description | What to Do? |
| ---- | ----------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| 200 | OK | The registration was successful. | No specific actions are required. |
| 401 | Unauthorized | The provider requests authentication. This response is expected on the first attempt as part of the SIP digest authentication handshake. | No action is required if the registration succeeds afterward. If the response repeats and the registration fails, check the [outbound authentication](/voice-gateway/webapp/carriers#outbound-authentication) credentials. |
| 403 | Forbidden | The provider rejected the registration. Possible causes include wrong credentials, an IP address that isn't allowlisted, or an account that the provider disabled or suspended. | Verify the username and password, check the provider's IP allowlist, or contact your provider. |
| 404 | Not Found | The provider doesn't recognize the user or address of record, for example, because of a wrong SIP URI or domain, or a typo in the username. | Verify the registrar domain and username with your provider. |
| 407 | Proxy Authentication Required | Authentication is required at a proxy, usually because of misconfigured outbound proxy authentication. | Check the proxy authentication credentials. |
| 408 | Request Timeout | The provider received the `REGISTER` request but timed out before completing processing, typically due to an overloaded or slow registrar. | Check the provider's status page. If the issue persists, contact your provider. |
| 423 | Interval Too Brief | The requested registration expiry is too short. The provider enforces a minimum expiry with the `Min-Expires` header. | Increase the `Expires` value to at least the provider's minimum. |
| 480 | Temporarily Unavailable | The provider can't process the registration temporarily, for example, due to a provider-side outage or maintenance. | Check whether registration succeeds after the outage or maintenance window ends. If it doesn't, check your provider's status page. |
| 500 | Server Internal Error | A failure on the provider side, usually a provider infrastructure issue. | Contact your provider support. |
| 503 | Service Unavailable | The provider's registrar is overloaded or down, for example, because of a provider outage. | Check whether registration succeeds once the provider's registrar is back up. If the issue persists, check your provider's status page or contact your provider. |
| 504 | Server Time-out | An upstream dependency of the provider failed or timed out. | Check whether registration succeeds after the upstream failure clears. If the issue persists, contact your provider. |
### Failures without a SIP Response Code
The codes in the table are SIP-level rejections, meaning that the provider actively responded to the REGISTER request. A registration attempt can also fail at the transport level, when the provider doesn't respond at all. In this case, the registration attempt appears as failed without a numeric SIP code. Typical causes include:
* A request timeout, where no response arrives from the registrar.
* A DNS failure when resolving the registrar hostname.
* A wrong registrar address or port in the carrier settings.
* A refused connection or a blocked SIP port `5060` or `5061`, for example, by a firewall.
To resolve these failures, verify the registrar host and port in the [carrier settings](/voice-gateway/webapp/carriers), and check the network path and firewall between Voice Gateway and the provider.
If you encounter any difficulties resolving your issue, contact [Cognigy technical support](/help/get-help).