SIP 180 Ringing vs 416 Unsupported URI Scheme
SIP 180 (Ringing) is a 1xx Provisional response, while 416 (Unsupported URI Scheme) 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, 416 means that the server cannot process the request because the URI scheme in the Request-URI is unknown to the server.
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 cannot process the request because the URI scheme in the Request-URI is unknown to the server.
When You See It
When using a URI scheme (e.g., tel:, im:, pres:) that the receiving server does not support.
How to Fix
Use a supported URI scheme. Most SIP servers support sip: and sips: schemes. Check server capabilities with OPTIONS.
Key Differences
180 is a 1xx Provisional response, while 416 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 416: The server cannot process the request because the URI scheme in the Request-URI is unknown to the server.
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 416 when when using a URI scheme (e.g., tel:, im:, pres:) that the receiving server does not support.
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 416 (Unsupported URI Scheme): Use a supported URI scheme. Most SIP servers support sip: and sips: schemes. Check server capabilities with OPTIONS.