SIP 180 Ringing vs 400 Bad Request
SIP 180 (Ringing) is a 1xx Provisional response, while 400 (Bad Request) is a 4xx Client 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, 400 means that the request could not be understood due to malformed syntax. The client should not repeat the request without modifications.
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 request could not be understood due to malformed syntax. The client should not repeat the request without modifications.
When You See It
When the SIP message has syntax errors, missing required headers, or invalid header values.
How to Fix
Review the SIP message for syntax errors. Check required headers (To, From, Call-ID, CSeq, Via, Max-Forwards) and ensure proper formatting.
Key Differences
180 is a 1xx Provisional response, while 400 is a 4xx Client 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 400: The request could not be understood due to malformed syntax. The client should not repeat the request without modifications.
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 400 when when the SIP message has syntax errors, missing required headers, or invalid header values.
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 400 (Bad Request): Review the SIP message for syntax errors. Check required headers (To, From, Call-ID, CSeq, Via, Max-Forwards) and ensure proper formatting.