What SIP provisional response is optional and used mainly for information w/o media?

Interview
1

Q. What SIP provisional response is optional and used mainly for information w/o media?

a) 305 Use Proxy
b) 182 Queued
c) 180 Ringing (w/o SDP)
d) 415 Unsupported

Check 🔎

:arrow_right: Correct answer: Letter c.

The 180 Ringing response is a SIP provisional response (1xx class) that indicates the called party is being alerted, i.e., the phone is ringing. It is commonly sent without an SDP body, especially in early-dialog SIP signaling, when the media path is not yet established or needed at that point.

It serves informational purposes only, letting the calling party know that the call is progressing without requiring immediate media negotiation.

Option Analysis:

a) :x: 305 Use Proxy
→ Incorrect. This is a redirection response (3xx), not a provisional (1xx) response.

b) :x: 182 Queued
→ Incorrect. While 182 indicates call is queued, it’s more commonly used when queuing mechanisms are implemented (e.g., call center scenarios). However, 180 Ringing is more universally used for non-media informational purposes.

c) :white_check_mark: 180 Ringing (w/o SDP)
→ Correct. This is a provisional response typically used without media, just to inform the caller that the callee is being alerted.

d) :x: 415 Unsupported
→ Incorrect. This is a 4xx class final response indicating unsupported media types — not provisional.