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What do the individual SIP messages mean?

You can find out what the individual SIP messages mean here:

1xx Informative feedback messages

100 Trying
This response indicates that action has been taken on behalf of the caller, but that the called program has not been located.

180 Ringing
This answer indicates that the called program has been located and the call is signalled.

181 Call is being forwarded
This answer indicates that the call is being diverted to another destination.

182 Queued
This answer indicates that the called program is currently unavailable, but the call is on hold and will not be rejected.

183 Session progress
This answer is used to perform in-band alerting for the caller.

2xx Successful responses

200 OK
This response indicates that the request has been successfully processed.

202 Accepted
The server has accepted the request, but postponed processing to a later date.

3xx Forwarding Feedback

300 multiple choices
Displays the address that has more than one address. All addresses are provided and the user or caller is allowed to choose which address to use.

301 Moved permanently
The user is no longer present at the specified position. A changing position is included in the heading.

302 Moved Temporarily
Indicates that the user is temporarily unavailable at the specified location. A changing position is captured in the heading.

305 Use proxy
The caller must use a proxy to connect to the called program.

380 Alternative service
The call was unsuccessful, but alternative services are available.

4xx Error message on request

400 Bad Request
The request could not be understood because it is based on an invalid unauthorized format.

401 Unauthorized
This request requires user authentication.
This is probably the most common error message that is issued if the login to our VoIP service fails. Please check your user name (phone number in international format) and password, which you can view in the customer portal. In isolated cases it may help to reset the password there.

402 Payment required
Indicates that a payment is required to make this call.

403 Forbidden
The server has understood the request, but unfortunately this service is not supported.

404 Not Found
The server has transmitted the information that the called party does not exist in the network.

405 Method Not Allowed
This feedback states that the method used is not permitted, but provides replacement options.

406 Not Acceptable
This call could not be put through because it was unacceptable.

407 Proxy authentication required
This feedback is almost identical to error 401 and indicates that the client must first log on to the proxy.

408 Request Timeout
This feedback means that the server could not establish a connection until the maximum time was exceeded.

409 Conflict
The request could not be processed due to a conflict with the current situation of the resource.

410 Gone
This announcement shows that a resource no longer exists at the operator and that no forwarding address is known.

411 Length required
This message indicates that the user refuses to accept the request without a defined content length.

413 Request entity too large
The server refuses the process because it is too long to process.

414 Request-URI too long
The server denies the process because it is too long to run.

415 Unsupported Media
The server has interrupted the process because the format is not supported when calling.

416 Requested Range Not Satisfiable / Unsupported URI Scheme
Both the start value and the end value of the specified range are outside the available byte range. The requested data is therefore not transferred.

420 Bad Extension
The server could not understand the log extension displayed in the required heading.

480 Temporarily unavailable
This feedback indicates that the called party is temporarily unavailable.

481 Call leg/transaction does not exist
This indicates that the operator is ignoring the request.

482 Loop detected
The server received the message that more jumps are required than allowed by the Max Forward.

484 Address incomplete
The server left the message that the address is incomplete.

485 Ambiguous
The server received a request in which the program address was ambiguous. It can provide possible alternate addresses.

486 Busy here
The caller's been contacted, but he seems busy.

487 Request terminated
This error occurs, for example, with the FRITZ!Box if no call is made after 60 seconds.

5xx Server error message

500 Server internal error
This message indicates that the server or gateway detected an unexpected failure that prevented the request from being processed.

501 Not implemented
The server or gateway does not support the functions required to complete the request.

502 Bad gateway
The server or gateway has received an invalid response from a downlink server.

503 Service unavailable
The server or gateway is unable to process the request due to congestion or a maintenance problem.

504 Gateway timeout
The server or gateway could not receive a timely response from the other server.

505 Version not supported
The server or gateway does not support the version of the SIP protocol used in the request.

6xx Global Responses

600 Busy everywhere
The called program has been questioned, but is busy and cannot accept the call this time.

603 Decline
The called program was questioned, but cannot or does not want to participate in the call.

604 Does not exist anywhere
The server has authoritative information that the called program does not exist in the network.

606 Not acceptable
The holder of the called number is currently unable to answer the call due to a power limitation.