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183
1xx Provisional, RFC 3261

SIP 183 Session Progress

SIP 183 Session Progress is not an error. The far network is sending audio before the call is answered, such as ringback, music or an announcement.

Who sends it
The called side
When
Ringing, before answer
Retry?
Nothing to retry. It is a progress message, not a failure.
The caller hears
Ringback, music or an announcement from the far network before answer, or silence if the early audio is blocked.
PBXYour phone or PBX
SIPThe SIP trunk
☎The called sidesends 183
FIX

What causes 183, and how to fix it

183 is how a carrier plays ringback tones and announcements before answer. Trouble starts when your system ignores the early audio or answers it with its own tone.

  1. 1
    Most common

    Your PBX plays its own ringing and mutes the early audio, so callers never hear announcements such as "number not in service".

    On outbound routes, let early media pass through instead of generating local ringback. In many PBXs this is called progress or early media.

  2. 2
    Most common

    A firewall or NAT blocks the audio until the call is answered, so callers hear silence before answer.

    Allow RTP from the trunk before the 200 OK, not only after answer, and turn off SIP ALG on the router.

  3. 3
    Sometimes

    The destination plays an announcement and then sends an error, so the call looks like it failed for no reason.

    Record or listen to the early audio. The announcement usually says what is wrong with the number.

  4. 4
    Rare

    The early audio uses a codec your phone handles badly, so it sounds garbled.

    Keep G.711 enabled on the phone and the PBX alongside any other codecs.

LOG

What 183 looks like in a SIP log

SIP/2.0 183 Session Progress

Capturing one: reading the SIP log.

SIP response
SIP/2.0 183 Session Progress
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK-74bf9
Call-ID: [email protected]
Content-Type: application/sdp
≠

Not to be confused with

183, 180, 200 or 487?

180

Ringing

The far phone is ringing but sends no audio, so your phone plays its own ringback. 183 carries real audio.

200

OK

The call was answered. Billing normally starts at 200 OK, not at 183.

487

Request Terminated

The call was cancelled before answer, often seen right after a 183.

FAQ

SIP 183 questions

Is SIP 183 an error?
No. It is a provisional response in the 1xx range. It means the call is progressing and early audio may be playing.
Am I charged during 183?
Normally not. Charging starts when the call is answered (200 OK). On LinkedSIP only answered calls are charged.
Why do callers hear silence before the call is answered?
Usually the early audio after a 183 is blocked by a firewall, NAT or SIP ALG, or the PBX mutes it. Allow RTP before answer and let early media through.

Not 183? Look up another code.

The full list: SIP response codes. 183 again and again? Check your SIP trunk configuration.

All SIP codesFind a fault by symptom