What every SIP response code from your PBX or softphone means, why it happens, and how to fix it — all on one page.
A SIP response code is a three-digit number your PBX, carrier, or softphone uses to tell you what happened to a call, defined in RFC 3261. They work like HTTP status codes: the first digit sets the category (1xx informational, 2xx success, 4xx client-side error, 5xx server-side error, 6xx global/permanent failure).
Below we list the most common codes in plain language — what each one means and what to do when you see it.
The server has received the request and is processing it — no result yet.
What to do: This is a normal intermediate state — no action needed.
The other party's phone is ringing; it hasn't been answered yet.
What to do: Normal — just wait for an answer.
The call isn't set up yet, but early media (like a ringback tone) is being sent.
What to do: Normal — this is usually why you hear a ringback tone at all.
The request (registration, call, hangup, etc.) completed successfully.
What to do: Nothing — everything is working correctly.
Incorrect username or password — the server is requesting authentication.
What to do: Verify your SIP account credentials (username, password) with your provider.
The server understood the request but refused it — usually an IP block, an unauthorized line, or a carrier restriction.
What to do: Ask your provider whether there's an IP restriction or line authorization issue.
The number or user you called couldn't be found on the server.
What to do: Double-check the number and area/country code you dialed.
Similar to 401, but here the intermediate proxy server is requesting its own authentication.
What to do: Check your proxy/PBX account settings.
The server didn't respond in time — could be network latency or a server issue.
What to do: Check your internet connection and provider status, then try again.
The called user can't be reached right now — their device is off or offline.
What to do: Try calling again later.
The server has no record of this call — usually a sync issue between the two sides.
What to do: Usually resolves itself; if it repeats, restart your PBX or softphone.
The SIP message got caught in a loop — usually a PBX or routing misconfiguration.
What to do: Have your PBX dial plan / routing configuration reviewed.
The called party is busy — currently on another call.
What to do: Normal — try calling again later.
The call was canceled before it was answered — usually the caller hung up.
What to do: Informational only — usually no action needed.
Audio codec mismatch — the two sides couldn't agree on a common audio format.
What to do: Make sure G.711 or Opus is enabled in your softphone/PBX codec settings.
A conflicting request for the same call is already being processed.
What to do: Usually retried automatically shortly after — no manual action needed.
An unexpected error occurred on the server side.
What to do: Not caused by you — report it to your PBX or carrier's support.
The server acting as a gateway/proxy received an invalid response from an upstream server.
What to do: Usually a temporary issue on the provider side — try again in a few minutes.
The server is temporarily overloaded or under maintenance.
What to do: Try again in a few minutes; if it persists, contact your provider.
The server didn't get a timely response from another server (e.g. DNS or an upstream server).
What to do: Usually a provider-side issue — try again.
The called party is busy on all of their devices.
What to do: Try calling again later.
The other party deliberately declined the call.
What to do: No action needed.
This number or user doesn't exist anywhere — a permanent condition.
What to do: Double-check the number — it's likely wrong or has been disconnected.
The other party accepted the call but rejected the proposed session parameters (codec, bandwidth, etc.).
What to do: Align your codec/audio settings with the other party's.
Emileyt V-Call automatically translates the SIP codes you encounter into plain Turkish/English sentences — the raw code stays in parentheses.
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