SIP 202 Accepted vs 501 Not Implemented
SIP 202 (Accepted) is a 2xx Success response, while 501 (Not Implemented) is a 5xx Server Failure response. 202 indicates that the request has been accepted for processing but the processing has not been completed. Used primarily for REFER requests. In contrast, 501 means that the server does not support the functionality required to fulfill the request. The method is recognized but not implemented.
Description
The request has been accepted for processing but the processing has not been completed. Used primarily for REFER requests.
When You See It
After sending a REFER request for call transfer. The transfer target acknowledges receipt but hasn't completed the transfer yet.
How to Fix
Wait for a NOTIFY to learn the outcome of the referred action. If no NOTIFY arrives, check subscription state and network connectivity.
Description
The server does not support the functionality required to fulfill the request. The method is recognized but not implemented.
When You See It
When the SIP method is valid but the server has not implemented it (e.g., INFO, UPDATE, PRACK on a basic server).
How to Fix
Use a different method that the server supports. Check the Allow header in responses to see supported methods.
Key Differences
202 is a 2xx Success response, while 501 is a 5xx Server Failure response.
SIP 202: The request has been accepted for processing but the processing has not been completed. Used primarily for REFER requests.
SIP 501: The server does not support the functionality required to fulfill the request. The method is recognized but not implemented.
You encounter 202 when after sending a REFER request for call transfer. The transfer target acknowledges receipt but hasn't completed the transfer yet.
You encounter 501 when when the SIP method is valid but the server has not implemented it (e.g., INFO, UPDATE, PRACK on a basic server).
When to Use Which
For 202 (Accepted): Wait for a NOTIFY to learn the outcome of the referred action. If no NOTIFY arrives, check subscription state and network connectivity. For 501 (Not Implemented): Use a different method that the server supports. Check the Allow header in responses to see supported methods.