SIP Response Codes

Code 180 and 183 are both SIP response codes used to indicate the progress of a call. While they may seem similar, they have distinct differences in their meanings and usage.

• The 180 Ringing response code indicates that the called party's device is being alerted or ringing.
• It is typically sent by the called party's User Agent (UA) to indicate that the call is being presented to the user.
  
• The 180 response code is often sent in response to an INVITE request, indicating that the called party's phone is ringing.
  
• The calling party may receive multiple 180 responses if the call is forwarded to multiple destinations.
  

• The 183 Session Progress response code indicates that the call is in progress, but the called party has not yet answered.
• It is typically sent by the called party's UA to indicate that the call is being connected or processed, but the called party has not yet accepted the call.
  
• The 183 response code is often used to indicate that the call is being connected to a voicemail system, an IVR, or a queue.
  
• Unlike 180, the 183 response code does not necessarily imply that the called party's phone is ringing.
  

Key differences:

• Ringing vs. Call Progress: 180 specifically indicates that the called party's phone is ringing, while 183 indicates that the call is in progress, but the called party has not yet answered.
• Device Alerting: 180 implies that the called party's device is being alerted, whereas 183 does not necessarily imply device alerting.
  
• Call State: 180 indicates that the call is in the "ringing" state, while 183 indicates that the call is in the "session progress" state.
  

In summary, while both 180 and 183 response codes indicate call progress, 180 specifically indicates that the called party's phone is ringing, whereas 183 indicates that the call is in progress, but the called party has not yet answered.