SIP 180 Ringing: what it means and how to fix it

Short answer: SIP 180 Ringing is a provisional response: the destination received the INVITE and is alerting the user, which means the phone is ringing. It is not a failure. The trouble starts when the 180 repeats and nothing follows it. In that case, check whether the call ends in 200 OK, 480 or 487.

How Monitor treats SIP 180

SIP 180 on its own is not a failure: the call is still in progress. What counts is how it ends. The table shows how a test call that rings until the test stops waiting is read for each kind of circuit, depending on what should answer the number.

What should answer the number	Reading
A greeting or recorded message (the IVR)	Down
A test tone or echo service	Down
Other: it may ring or be answered	Up

For "Other": With "Count ringback as reachable" unticked: Down.

What Monitor records for an IVR: The line rang and nothing answered, but this circuit is supposed to answer every call by itself.

A call read as Down alerts nobody on its own: it is repeated 3 times, every 2 minutes, and only 4 failures in a row open an incident, with each call's SIP code and Q.850 cause as evidence for the carrier. At the default pace, every circuit is tested every 15 minutes.

What does SIP 180 mean in practice?

SIP 180 Ringing is the provisional response that says “the destination is ringing.” Under RFC 3261, the agent that received the INVITE is trying to alert the user, and the calling side can use this signal to generate the ringback tone locally. The 180 creates a provisional dialog (early dialog), but it does not settle anything: the call is still waiting for a final response, which can be 200 OK (answered), a 4xx, 5xx or 6xx error, or a CANCEL from the caller.

That is why a 180 on its own is not a diagnosis. It tells you the signaling reached something capable of ringing. What matters to anyone monitoring a trunk or an 0800 number is what happens after the 180: whether the destination answers, how long it takes, and whether the call keeps ringing forever without anyone, not even the IVR, picking up.

Common causes in Brazil

• An extension or IP phone ringing normally on an Asterisk, Issabel, FreePBX, 3CX or Intelbras PBX: expected behavior.
• An 0800 number or IVR that should answer right away but keeps ringing because the IVR server froze and the PBX keeps alerting a destination that does not respond.
• A call-center queue with no agents logged in, set up to ring until someone joins instead of rejecting the call or sending it to a message.
• A wrong time-of-day rule on the PBX that sends after-hours calls to a group of extensions nobody answers.
• An interconnection with the phone network where the carrier passes the destination’s alerting (mobile or landline) through as a 180.
• No ringback tone for the caller, because the 180 arrived without SDP and the originating equipment does not generate the tone locally.

How to diagnose it

1. Capture the signaling with sngrep (for example, sngrep port 5060) and check the sequence: INVITE, 100 Trying, 180 Ringing, and which final response comes after.
2. On Asterisk, Issabel or FreePBX, turn on pjsip set logger on (or sip set debug on with chan_sip) and repeat the call. Check whether the 180 carries an SDP body.
3. Use core show channels during the call to see which extension, queue or application the channel is sitting on while it rings.
4. On 3CX, enable “Verbose logging” and read the call’s activity log to see which extension or queue it was delivered to.
5. Measure the time between the 180 and the final response. If the call ends in 487 Request Terminated, the calling side gave up. If it ends in 480, the destination ended it.
6. If the 180 repeats many times, check whether the destination uses reliable provisional responses (Require: 100rel and PRACK, RFC 3262) and whether the PRACKs are arriving.

What to send the carrier

• Date and time of each attempt, with the time zone (for example, Brasília time, UTC-3).
• Calling number (A) and called number (B), in the exact format sent in the INVITE.
• The Call-ID of at least one affected call.
• The full sequence of responses: how many 180s, whether a 183 came, and the final response line (for example, SIP/2.0 480 Temporarily Unavailable).
• The Reason header, if present, with the Q.850 cause (Reason: Q.850;cause=19).
• The pcap trace produced by sngrep or tcpdump, when available.

Frequently asked questions

Why do I get 180 Ringing but hear no ringback tone?

When the 180 arrives without SDP, your side is expected to generate the ringback tone locally. If the PBX or the phone does not generate it, the call stays silent until it is answered. If the destination sends its own audio, the usual approach is 183 Session Progress with SDP. Check the ringback settings on the trunk and the extension.

Is a call that rings and nobody answers a problem?

It depends on the destination. A personal extension may simply go unanswered. An 0800 (toll-free) number or an IVR, on the other hand, should always answer. If the 180 repeats and the call ends in 480 Temporarily Unavailable or is canceled by the calling side (487), the automated answering has stopped working, or the queue has no free positions.

Does a real test call notice when the 0800 just rings?

Yes. The test call is a real call, so it sees the sequence of SIP responses all the way to the final response, not just the first 180. A 180 on its own does not decide the result. When the test fails, it is repeated before any alert goes out, to tell a real problem apart from a passing blip.