SIP 486 Busy Here: what it means and how to fix it

Short answer: SIP 486 Busy Here means the destination was reached but cannot accept another call at that point. On an extension or mobile phone, the person is already on a call. On a SIP trunk, the most common cause is the concurrent-channel limit being used up. Check how many calls were active at the time and compare that with the contracted limit.

How Monitor treats SIP 486

The reading depends on what should answer the number, which is the question 2N5 answers when it adds each circuit. These are the rules the product applies today, generated from the same function that judges every test call.

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 busy as reachable" unticked: Down.

What Monitor records for an IVR: The line was busy: the phone is on another call, but this circuit is supposed to answer every call by itself (SIP 486).

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.

If the carrier returns SIP 486 when 2N5's calling account registers, before any call, the test is monitoring blind: it is a problem on 2N5's side, it does not count against the circuit and it does not alert the customer.

What does SIP 486 mean in practice?

SIP 486 Busy Here is a final 4xx-class response: the destination’s system was contacted, but it will not or cannot accept another call at that point. RFC 3261 lets the response carry the Retry-After header, suggesting when to try again. When converting to the phone network, RFC 3398 maps the 486 to Q.850 cause 17 (User busy), and the reverse mapping also leads to 486.

On an ordinary line, “busy” means someone is already talking. On a SIP trunk, the meaning is usually different: all the concurrent channels contracted or configured are in use, and the next call is refused as busy. That is why a SIP 486 error at peak hours deserves attention even when nobody is on the phone. Unlike SIP 600, the 486 applies only to that destination; another point may still answer.

Common causes in Brazil

• Concurrent channels used up on the SIP trunk contracted with the carrier (Vivo, Claro, Algar, Embratel or another), usually at peak hours or during an outbound calling campaign.
• A channel limit configured on the PBX itself (Asterisk, Issabel, FreePBX, 3CX) lower than the contracted one, refusing calls before the trunk is full.
• A destination extension or mobile phone already on a call, with no call waiting enabled.
• An IVR’s queue or answering group with no free agents, set to return busy instead of keeping the call in the queue.
• “Do Not Disturb” turned on for an Intelbras extension, IP phone or softphone, which many devices signal as 486.
• Stuck calls: channels that were not released after a failed BYE still count as in use.

How to diagnose it

1. Capture the signaling with sngrep on the server and see who generated the 486: the PBX, the SBC or the carrier. The response’s Via, Server and User-Agent headers help identify it.
2. On Asterisk, Issabel or FreePBX, run core show channels at the moment of the failure to count the active calls and compare them with the trunk limit.
3. Turn on pjsip set logger on (or sip set debug on with chan_sip) and look for Reason: Q.850;cause=17 and any Retry-After in the response.
4. Look for stuck channels: calls with a very long duration in core show channels are usually sessions that never got a BYE.
5. On 3CX, enable detailed (Verbose) logging and check in the Activity Log whether the refusal came from the trunk or from an internal rule.
6. Call the same number through another trunk or from a mobile phone. If it goes through, the problem is the route’s capacity, not the destination.

What to send the carrier

• Exact date and time with the time zone, for example 2026-10-07 14:32:10 -03:00.
• Calling number (A) and destination number (B), in the format sent in the INVITE.
• The Call-ID of the call and the full response line, such as SIP/2.0 486 Busy Here.
• The Reason header with the Q.850 cause, if it came, and the Retry-After value.
• How many attempts failed, over what interval, and how many calls were active on the trunk at that moment.
• The pcap file of the capture, if you have one.

Frequently asked questions

What is the difference between SIP 486 and SIP 600?

486 Busy Here says the destination is busy at that point, and another device or another route may still answer. 600 Busy Everywhere is a global failure: the destination knows no other point will accept the call, and the proxy should not try other routes. On trunks and PBXs in Brazil, the 486 shows up far more often.

Why does the trunk return busy when nobody is on the phone?

Because on a SIP trunk 'busy' usually means there is no free channel. If the contract allows, for example, 30 concurrent calls and all of them are in use, the next one may get a 486. Some SBCs and carriers use SIP 503 with Q.850 cause 34 in this same situation. Compare your peak call count with the contracted limit.

Does a real test call get this busy signal?

Yes. A real test call goes through the trunk like any other call, so it gets the final SIP response the network returns, including the 486. When the result counts as a failure, the call is repeated before any alert goes out, and the alert is sent by email and SMS.