SIP 480 Temporarily Unavailable means the called user was found but cannot take the call right now, for example because the phone is off or not answering. SIP 408 Request Timeout means no final response arrived in time, and SIP 486 Busy Here means the line is busy. All three are usually about the called person or the path to them, not a broken route, but a sudden rise in any of them is worth checking.
What are SIP 408, 480 and 486 at a glance?
| Code | RFC 3261 meaning | Common Q.850 cause (RFC 3398) | Usual reason on a VoIP route |
|---|---|---|---|
| 408 Request Timeout | No response produced within a suitable time | 18 No user responding | Phone unreachable, slow or dead next hop |
| 480 Temporarily Unavailable | Callee contacted but currently unavailable | 19 No answer, 20 Subscriber absent, 31 Normal unspecified | Mobile off or out of coverage, no answer, do not disturb |
| 486 Busy Here | Callee not willing or able to take more calls at this end system | 17 User busy | Line busy, call waiting off, user rejected |
The Q.850 column shows the usual mapping when a call crosses from the telephone network to SIP. Carriers do not all follow it, so read the Reason header and your Q.850 cause codes together with the SIP code.
What does SIP 480 Temporarily Unavailable mean?
480 means the far end recognised the called user but cannot reach them now. RFC 3261 gives examples such as a user who is not logged in or has set do not disturb, and says a proxy can also return 480 when it knows the user but has no current location for them. On mobile destinations it typically means the handset is switched off, out of coverage or did not answer within the network timer.
Some carriers also use 480 for "no route available" or for calls blocked by their filtering. If 480s jump on one destination while other routes to the same numbers connect, the supplier is the likely cause.
What does SIP 408 Request Timeout mean?
408 means a server could not produce a final response in time. It can come from the far end (for example, the network could not locate the user before its timer expired) or it can be generated by your own equipment. Under RFC 3261, a client transaction gives up when Timer B fires, by default 64 times T1, which is 32 seconds with the standard 500 ms T1, and reports a 408.
- A locally generated 408 with no response at all from the provider usually means a network, firewall or NAT problem: your INVITE never reached the provider or the reply never came back.
- A 408 received from the provider after ringing usually means the called phone did not respond.
What does SIP 486 Busy Here mean?
486 means the called end system was reached but is not willing or able to take another call there. Most often the person is on another call. Some phones also send 486 when the user presses reject. 486 differs from 600 Busy Everywhere, which says the user is busy on all their devices.
Real busy rates depend on the country and time of day. A route that returns far more 486s than another route on the same numbers may be returning fake busy signals instead of completing calls, which is worth raising with the supplier.
Why do outbound calls get SIP 404 Not Found?
404 is often confused with 480. It means the server is sure the number does not exist at that domain. On outbound calls the cause is nearly always the number format: a missing country code, an extra national 0, or a technical prefix the supplier did not expect. Send numbers in E.164 format and agree any prefix in writing. The full list is on our SIP error codes page.
How do these codes affect ASR and NER?
All three lower ASR, because the call was not answered. NER treats busy and no answer as normal user outcomes, so a route with many 486s and 480s can still have a good NER (see NER vs ASR). A healthy route shows stable shares of each code from day to day. Track them per destination in your CDRs and compare suppliers on the same traffic before you conclude anything. If one supplier's 480 or 408 share stays well above the others, VoIPMatcher can source an alternative from its carriers: see our VoIP termination service and request a VoIP route.
Frequently asked questions
Why do I get SIP 480 on every outbound call?
If every call fails with 480, the called users are not the problem. Common causes are a supplier with no working route for the destination, a blocked or unrecognised caller ID, or an account restriction. Test the same numbers on another route and send a SIP trace to the supplier with the exact time of the failed calls.
Why do my SIP calls time out after 32 seconds?
32 seconds is the default SIP Timer B (64 times a 500 ms T1). If no final response arrives within it, the calling side ends the attempt with a 408. Consistent 32 second timeouts with no reply at all usually mean the INVITE or the response is lost, often because of a firewall, NAT or wrong IP.
What is SIP 600 Busy Everywhere?
600 Busy Everywhere means the called user is busy on every device where they could be reached, and the caller should not try other locations. It is a global failure, unlike 486 Busy Here, which only refers to the endpoint that answered. It is rare on wholesale routes, where 486 is the usual busy signal.
Can a route fake busy signals?
Yes. Some low quality routes return busy or unavailable responses instead of completing calls, which lowers ASR without obvious errors. Compare the share of 486 and 480 responses between suppliers on the same numbers and times. A route with far more busy results than its peers deserves a closer test with real handsets.
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