SIP 180 Ringing vs 501 Not Implemented
SIP 180 (Ringing) is a 1xx Provisional response, while 501 (Not Implemented) is a 5xx Server Failure response. 180 indicates that the UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end. 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 UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end.
When You See It
During call setup when the callee's phone is ringing. You should hear a ringback tone after receiving this response.
How to Fix
No fix needed — the callee's device is ringing. If the call never connects after ringing, check the callee's device or call forwarding settings.
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
180 is a 1xx Provisional response, while 501 is a 5xx Server Failure response.
SIP 180: The UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end.
SIP 501: The server does not support the functionality required to fulfill the request. The method is recognized but not implemented.
You encounter 180 when during call setup when the callee's phone is ringing. You should hear a ringback tone after receiving this response.
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 180 (Ringing): No fix needed — the callee's device is ringing. If the call never connects after ringing, check the callee's device or call forwarding settings. For 501 (Not Implemented): Use a different method that the server supports. Check the Allow header in responses to see supported methods.