In a VoLTE MO call, what provisional response often carries early media SDP?

Interview
1

Q. In a VoLTE MO call, what provisional response often carries early media SDP?

a) 100 Trying
b) 180 Ringing
c) 302 Moved
d) 183 Session Progress

Check 🔎

:arrow_right: Correct answer: Letter d.

In VoLTE (Voice over LTE), the 183 Session Progress response is used to convey early media, such as announcements, tones, or ringback audio before the call is fully answered. This SIP provisional response typically includes an SDP (Session Description Protocol) body that allows the establishment of the media path early in the call setup.

This is particularly critical in VoLTE for pre-answer audio interactions with the user (e.g., call progress tones or announcements from IVR systems).

Option Analysis:

a) :x: 100 Trying
→ Incorrect. It’s purely informational and does not carry any media or SDP.

b) :x: 180 Ringing
→ Incorrect. It can carry SDP, but it’s not typically used for early media — mostly just to indicate that the callee is being alerted.

c) :x: 302 Moved
→ Incorrect. This is a redirection response, unrelated to provisional signaling or early media.

d) :white_check_mark: 183 Session Progress
→ Correct. This is the standard SIP response for early media, often used in VoLTE MO calls with SDP. 183 with SDP enables resource reservation and ringback media.