SIP Response Codes and What They Mean

SIP Response Codes

There are new protocols in place but good old SIP or session initiation protocol still plays a significant role in VoIP telephony. SIP is in charge of handling sessions of telephony over the internet. The way it works, a request is sent out in a SIP session and a SIP response is received through the SIP user agents comprising of user agents on the client-side and on servers for the call receiving side. The roles switch depending on whether one is making or receiving a call. Devices can take on both functions. A SIP request generates a response that comprises header fields and the response falls into six categories, broken down into provisional and final SIP responses.

The SIP response categories classes:

Each SIP session starts with requests and responses and these are classified in order to help users to know the status and possible errors.


    • 1xxx class is informational making up provisional code
    • 2xx denotes success
    • 3xx stands for redirection
    • 4xx typifies client error
    • 5xx indicates a server error
    • 6xx series is an SIP response for global failure.

2xx through 6xx are final codes to indicate whether a connection is successful or not. Each code has a string in attendance to give you a tip about the possible fault so that you can take appropriate action. Errors may occur on the transmitting side or receiving side or due to the intermediate pathway.

The 1xx code

The system generates 1xx code while a connection is being established. You can get any of these messages:

    • 100: signals that the system is trying to establish a connection
    • 180: The caller agent has dialed the called number and that number is ringing
    • 181: Call is being forwarded to another endpoint
    • 183: A session is established and is under progress
    • 199: SIP entities receive notification that a dialog has been ended.

The 2xx code

The 2xx code denotes success in initiating a SIP request and show it has been received and is accepted.

    • 200: Request is successful
    • 202: The receiving side has received the request but it may not yet have been authorized or been processed.
    • 204: There is no notification despite the request being successful.

The 3xx code

The caller is notified about redirections through the 3xx SIP response codes.

    • 300: Request generates several options and the calling user agent can pick a preferred endpoint to redirect the request to that location.
    • 301: This response shows the called party is not available at the defined address. A new address may be given to help you start a new SIP session.
    • 302: Indicates addressee has moved temporarily and you may use the new address in the Contact header field to request a connection.
    • 305: You will need to use a proxy to access the called party, as shown in the contact field.
    • 380: The call did not go through but you are shown options.

The 4xx code

This series indicates the non-fulfillment of SIP session requests.

    • 400: Request is bade and could not be understood
    • 401: User request needs to be authenticated
    • 404: Similar to HTTP, this shows server has no information about the user in that domain
    • 407: Client requires to authenticate with the proxy
    • 408: Request has been timed out
    • 415: Message is not in the desired format or is not support and, hence, rejected due to unsupported media type

The 5xx code

Server errors fall under this series and are generated by redirect servers, proxies, and location servers.

    • 500: Internal server error prevents the fulfillment of a SIP request
    • 501: Server does not support functionality to fulfill the request
    • 502: The server receives an invalid response from a downstream server and displays a Bad Gateway message.
    • 503: If the server is overloaded or under maintenance then it displays a 503 Service Unavailable message.
    • 504: The external server did not receive a SIP response in time from the external server and hence it shows Server Time Out.

The 6xx code

1 through 5 are similar to HTTP but 6xx is specific to SIP and related to global errors.

    • 600: Called party’s system was contacted successfully but the called party is busy and shows Busy Everywhere message.
    • 603: Called party’s system connection was successful but called party declines to take the call and displays Decline message.
    • 604: The user requested in the URI request does not exist in the list and shows a Does Not Exist Anywhere message.
    • 606: Called party wishes to communicate but their system may not support the session and this displays a Not Acceptable message.

Given the complexities of SIP trunking and the way the internet and VoIP telephony operate, you are likely to see these messages. Knowing about SIP responses and SIP requests will help you gain a clearer idea about what the messages stand for and what action you could possibly take.

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