SIP 480 Temporarily Unavailable: what it means and how to fix it

Short answer: SIP 480 Temporarily Unavailable means the destination was located but is not available to take the call right now: it is not registered, it is on Do Not Disturb or it did not answer. The most common cause on a PBX is an extension or IP phone that is not registered. Check the destination's registration and look for the Retry-After header, which may suggest when to try again.

How Monitor treats SIP 480

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	After ringing	Without ringing
A greeting or recorded message (the IVR)	Down	Down
A test tone or echo service	Down	Down
Other: it may ring or be answered	Up	Down

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

What Monitor records for an IVR: The line rang and nobody answered, but this circuit is supposed to answer every call by itself (SIP 480).

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 480 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 480 mean in practice?

Under RFC 3261, SIP 480 Temporarily Unavailable indicates that the destination’s system was contacted, but the user is not available at the moment: not logged in, in a state that prevents communication, or with Do Not Disturb turned on. The response may carry the Retry-After header with a better time to call. Proxies and redirect servers also use 480 when they recognize the user but have no valid address to deliver the call to.

In telephony, the 480 also carries network situations: Q.850 cause 20, such as a mobile phone switched off or out of coverage, cause 19, when the destination rang and nobody answered, and Q.850 cause 31. It is a “temporary” code: the same number may go through minutes later. A SIP 480 error that keeps repeating on the same destination, however, deserves a closer look.

Common causes in Brazil

• An extension or IP phone not registered on Asterisk, Issabel, FreePBX, 3CX or Intelbras, because of a network outage, a restart or a changed password.
• An extension with Do Not Disturb turned on, set to reject the call instead of sending it to voicemail.
• A destination mobile phone that is switched off, has no signal or is out of the coverage area.
• A call that rang until the PBX’s or the network’s time limit with no answer.
• An IVR or auto-attendant server that lost its registration with the PBX, leaving the 0800 number with no destination.
• A queue with no available agents, set to end the call instead of keeping it on hold.

How to diagnose it

1. Capture with sngrep port 5060 and see who generated the 480: the carrier or your PBX. Check the Reason: Q.850;cause=... and Retry-After headers.
2. On Asterisk or FreePBX, run pjsip show contacts (or sip show peers with chan_sip) and confirm whether the destination extension is registered and responding.
3. Turn on pjsip set logger on (or sip set debug on) and repeat the call to see whether it rang (180 or 183) before the 480.
4. Run core show channels during the call to see which queue, group or extension it stopped at.
5. Review the PBX’s Do Not Disturb, time-of-day and queue rules for the affected destination.
6. On 3CX, enable “Verbose logging” and check the status of the extension or queue at the time of the call.

What to send the carrier

• Date and time of the calls, with the time zone.
• Calling number (A) and called number (B).
• The Call-ID of at least one call.
• The full 480 line, with the Reason (Q.850 cause) and Retry-After headers, if present.
• Whether the call rang and how many attempts failed.
• The pcap of the capture, when available.

Frequently asked questions

Which Q.850 cause comes with a 480?

Under RFC 3398, three causes are converted into 480: 19 (No answer from user, the destination rang and was not answered), 20 (Subscriber absent, such as a mobile phone switched off or out of coverage) and 31 (Normal, unspecified). In the other direction, a 480 entering the phone network becomes Q.850 cause 18 (No user responding).

Does a 480 mean my 0800 number is down?

For an 0800 (toll-free) number or an IVR, which should always answer, a 480 is a sign of trouble: the final destination is not available, either because the IVR server dropped its registration or because the queue had nobody to answer. For a personal extension, it may just be someone on Do Not Disturb or away from their desk.