What Do SIP Codes Like 200, 404 or 486 Mean?

Definition and origin

The SIP protocol, which manages calls over the internet, borrowed many ideas from the protocol of websites (HTTP). One of them is response codes. Anyone who has ever run into a "404" page on a website already knows the principle: a fixed number that says what happened to the request.

The codes are defined in SIP's core standard, RFC 3261, and in the standards that followed. Next to the number there is also a word or two in English, such as "Busy Here" or "Not Found", but systems rely on the number. The words are for humans.

For an ordinary user, the codes appear mainly where something has failed: a message on the phone's display, a softphone's log, or the call details in the PBX log.

The codes are a common language worldwide. A phone from one manufacturer, a PBX from a second and a provider in a third country all understand that 486 means "busy". That is why equipment from different manufacturers can be connected without special translation.

The six families

The first digit says which family the code belongs to. Remembering it is enough to understand the direction:

A rule of thumb: 4xx, check the request (number, password, permission). 5xx, check the server or the provider. 6xx, the destination decided not to accept.

There are also codes that are not in the standard: some manufacturers add their own codes, usually within the same families. If you run into an unfamiliar code, the first digit still tells you which direction to look.

A regular call in codes

To understand the codes, it helps to see what they look like when everything works:

1. The phone sends a request to dial (INVITE).
2. The PBX answers 100 Trying: "Got it, working on it."
3. The other side starts ringing, and sends back 180 Ringing. On your end you hear a ringing tone.
4. Sometimes 183 Session Progress arrives instead: the other side plays something itself, such as a message or music, even before anyone answered.
5. Someone answers, and 200 OK comes back. The call begins.
6. When someone hangs up, a hang-up request (BYE) is sent, and the other side answers 200 OK again.

All your calls go through these stages, within seconds, without you seeing them. Only when something goes wrong does the code that appears in place of one of them tell you what happened.

Registration codes: 200, 401, 403

When a phone registers, three codes appear most often:

• 200 OK: the registration succeeded.
• 401 Unauthorized: "Please identify yourself." At the start of a registration this is completely normal: the PBX sends a challenge, and the phone answers with the password. Only if 401 comes back again and again is the username or password wrong. A close code, 407, means the same thing when authenticating against an intermediate server.
• 403 Forbidden: "Not allowed." The authentication was understood, and the answer is no. It can be a wrong password, a blocked extension, or a registration from a place that is not authorized.

The important difference: 401 is "Who are you?", 403 is "I know who you are, and you are not allowed."

Another code worth knowing in registration is 423 Interval Too Brief: the phone asked for too short an expiry time, and the PBX asks for a longer one. Phones fix this by themselves, and there is nothing to do.

Dialing codes: 404, 408, 480, 484, 486

Note 487: it looks like an error, but in most cases it is simply a record that "the caller hung up before anyone answered." In the call history, it is one of the most common codes for unanswered calls.

Server codes: 500, 503

• 500 Server Internal Error — something went wrong on the server itself. Usually temporary; if it keeps happening, report it.
• 503 Service Unavailable — the server or the provider cannot handle the request right now: heavy load, maintenance, or the outbound route is unavailable. In systems with a backup, 503 is usually the sign to try a different route.

If a 5xx appears on a single call, it is probably a momentary glitch. If it appears on all outgoing calls, it is a wider problem, and it is worth contacting support right away, with the time and an example of a number that was dialed.

Some codes say "I passed the request along, and no answer came back," such as 502 Bad Gateway and 504 Server Time-out. They hint that the problem is not in your phone or in the PBX itself, but in a link further along — for example, with a provider along the way.

Audio-related codes: 488 and 415

Some codes do not say "not found" or "busy," but "we did not agree on how to talk":

• 488 Not Acceptable Here — the two sides did not find a shared codec, or the audio settings did not match. It happens, for example, when a device is set to only one codec that the other side does not support.
• 415 Unsupported Media Type — the request was sent in a format the other side does not recognize.

These codes are rare in everyday use, but when they do appear, they point straight to the device settings, not to the network.

Redirects and rejections: 3xx and 6xx

• 301 / 302 — "the destination has moved." The server returns a different address, and the phone is supposed to try there. In cloud PBXs, users almost never see this, because the transfers happen inside the PBX.
• 603 Decline — the destination explicitly rejected the call, for example when someone pressed "Decline" on a mobile phone, or a number was blocked.
• 600 Busy Everywhere — busy everywhere the destination was tried.

The difference between 486 and 603 is small to the eye but important: 486 is "busy right now," 603 is "does not want to receive." What the PBX does after each one can be different.

Summary table: what you see and what to do

The table does not cover every code, but it covers most of what you will actually run into.

Where to see the codes

• On the phone display — some phones show the code or its name when a call fails.
• In the phone's log — in the management interface of most IP phones there is a status page or a message log.
• In a softphone — in many apps, in the settings screen or the "log."
• In the PBX — the call history sometimes shows the reason each call ended.

Tip: when you report a problem, a photo of the display showing the code, or a copy of the line from the log, saves a lot of questions.

An example from the field

At the "Shaarei Horaah" institute (a made-up name), a new employee installed a softphone on her mobile phone, and it kept showing "Registration failed: 403." She typed the password again and again, and nothing worked.

It turned out she had copied the control panel password instead of the extension's password. 403 said exactly that: "I understand who you are, and the password does not match." After the correct password was copied from the extension details, a 200 OK came back and the extension registered within a second.

SIP codes versus what you hear on the phone

Someone calling from a mobile phone does not see codes. They hear a tone or a message. The PBX and the providers translate the code into something a person understands:

The practical meaning: in a well-configured PBX, most of the "negative" codes do not reach the caller as an error tone, but are translated into an action — a voicemail box, a queue, a transfer to another extension. That is exactly what a good call flow does.