SIP 181 Call Is Being Forwarded vs 183 Session Progress
Both SIP 181 (Call Is Being Forwarded) and 183 (Session Progress) belong to the 1xx Provisional category. 181 indicates that a proxy server may send this to indicate that the call is being forwarded to a different set of destinations. Meanwhile, 183 means that used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
Descrição
A proxy server may send this to indicate that the call is being forwarded to a different set of destinations.
Quando você o vê
When the callee has call forwarding enabled and the request is being redirected to another number or device.
Como corrigir
No action needed. The call is being redirected. If forwarding is unintended, the callee should update their forwarding settings.
Descrição
Used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
Quando você o vê
During call setup, especially in PSTN interworking scenarios. The response may contain SDP for early media like custom ringback tones or IVR prompts.
Como corrigir
No fix needed — this is informational. If early media is not playing, check codec negotiation and SDP handling in your UA.
Diferenças principais
SIP 181: A proxy server may send this to indicate that the call is being forwarded to a different set of destinations.
SIP 183: Used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
You encounter 181 when when the callee has call forwarding enabled and the request is being redirected to another number or device.
You encounter 183 when during call setup, especially in PSTN interworking scenarios. The response may contain SDP for early media like custom ringback tones or IVR prompts.
Quando usar qual
For 181 (Call Is Being Forwarded): No action needed. The call is being redirected. If forwarding is unintended, the callee should update their forwarding settings. For 183 (Session Progress): No fix needed — this is informational. If early media is not playing, check codec negotiation and SDP handling in your UA.