Protocols & Standards
SIP Response Codes
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
SIP response codes are the numbers phone systems send each other to explain what happened to a call. The first digit tells you almost everything, because it says whether the call failed at your end, at the far end, or somewhere in between.
How it works
Every SIP message gets a numbered reply, borrowed in style from the web's familiar 404. The first digit sets the category. A 1xx code means something is in progress, 2xx means success, and 3xx is a redirect. A 4xx code means your side sent something the far end would not accept, 5xx means a server had a problem, and 6xx is a refusal that applies everywhere.
That grouping is the practical value. A 4xx points at your configuration, your credentials or the number you dialed. A 5xx points at the provider's equipment. Before reading the specific number, the first digit has already told you who is likely to be able to fix it.
Why it matters
These numbers turn a vague complaint into a specific question. "Calls to that customer fail" is a support ticket that goes nowhere for three days. "Calls to that number return 404 Not Found" gets an answer, because it says the number does not exist on the route being used rather than that anything is broken.
The codes worth memorising are few. A 403 Forbidden almost always means credentials or permissions, commonly international calling switched off, which is also what a provider does when it suspects toll fraud. A 404 Not Found means the number does not exist on that route. A 486 Busy Here means the far end is genuinely busy. A 408 Request Timeout means nothing answered at all, usually a network path problem. And a 503 Service Unavailable means the provider's equipment is refusing, often capacity or an account issue.
1xx in progress · 2xx success · 3xx redirected · 4xx your request was rejected · 5xx the far-end server failed · 6xx refused everywhere. A 4xx is usually yours to fix; a 5xx usually is not.
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What does SIP 403 Forbidden mean?
The far end understood the request and refused it, which is nearly always permissions rather than a fault. The usual causes are wrong credentials, calling a destination your account is not allowed to reach, or international dialing being switched off. Providers also return it when they have blocked a route after suspecting toll fraud.
What does SIP 486 Busy Here mean?
The person you called is on another call and has no call waiting, so it is not an error at all. If you see it constantly for a number that should be free, look at how many simultaneous calls that extension or trunk is allowed — hitting the limit produces the same code.
What is the difference between 404 and 503?
404 Not Found means the number does not exist on the route used, which is usually dialing or routing configuration. 503 Service Unavailable means the provider's equipment could not take the call, from capacity, an account problem, or an outage. The first is generally yours to fix; the second is theirs.
Where do I find these codes?
In your phone system's call logs or CDRs, usually in a column labeled disposition, cause or SIP code. Hosted systems vary in whether they show them, and if yours does not, support can look them up. Quoting the code turns a vague ticket into a specific one.