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503
5xx Server error, RFC 3261

SIP 503 Service Unavailable

SIP 503 Service Unavailable means a server on the call path cannot take the call right now: it is overloaded, in maintenance, or has no route with free capacity. It is usually temporary and rarely caused by your settings.

Who sends it
The SIP trunk
When
Placing the call (INVITE)
Retry?
Yes, once, after a pause. Wait for the Retry-After time if the response gives one.
The caller hears
A fast busy tone or a short failure message. Some phones show "Service Unavailable" on the screen.
PBXYour phone or PBX
SIPThe SIP trunksends 503
☎The called side
FIX

What causes 503, and how to fix it

503 means the server or route cannot take the call right now. Many systems also use it when no route has capacity, so it is common during busy periods or outages.

  1. 1
    Most common

    Your PBX lost its registration with the trunk, so it answers your phone with a 503 itself.

    Open the trunk status in your PBX. If it says unregistered, check the server, username and password, then reload the trunk.

  2. 2
    Most common

    The route to that destination has no free capacity at this moment.

    Call a different country. If only one destination fails, wait a few minutes, retry once, then report the number, the time and the Call-ID.

  3. 3
    Sometimes

    You reached the limit on calls at the same time for your login or account.

    Count the calls that were up when it failed and compare them with your call limit. Ask for a higher limit if you need it.

  4. 4
    Sometimes

    The provider has maintenance or an incident.

    If every call fails at once and nothing changed on your side, check the provider's status updates before you touch your settings.

  5. 5
    Rare

    Fast automatic retries got your system rate limited.

    Lower the retry attempts on the trunk and respect Retry-After, so one failure does not turn into many.

LOG

What 503 looks like in a SIP log

SIP/2.0 503 Service Unavailable

Capturing one: reading the SIP log.

SIP response
SIP/2.0 503 Service Unavailable
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK-74bf9
Call-ID: [email protected]
Retry-After: 30
Reason: Q.850;cause=34;text="No circuit/channel available"
≠

Not to be confused with

503, 500, 480, 486 or 408?

500

Server Internal Error

A fault inside a server. 503 is about capacity or maintenance, 500 about something breaking.

480

Temporarily Unavailable

The person's phone is offline. With 480 the network is fine and the callee is unavailable.

486

Busy Here

The person you called is busy. 503 is the network, 486 is the human.

408

Request Timeout

No answer arrived at all in time. 503 is an answer that says try later.

FAQ

SIP 503 questions

Is a SIP 503 my fault?
Rarely. It is usually a capacity, maintenance or route problem upstream. The main exception is your own PBX sending 503 to your phones because its trunk is down, which the trunk status page will show.
Should I retry after a 503?
Once, after a pause. RFC 3261 says that without a Retry-After header a client should treat 503 like a 500 server error, and that a proxy may try another server. Rapid repeated retries make things worse.
What does Retry-After mean on a 503?
The number of seconds the server asks you to wait before trying again. Your PBX should wait at least that long before sending the call again.
Why does my log show Q.850 cause 34 with a 503?
Cause 34 means no circuit or channel is available. Gateways map it to 503 when a call crosses into the phone network (RFC 3398), so it points to a full route rather than a setting on your side.

Not 503? Look up another code.

The full list: SIP response codes. 503 again and again? Check your SIP trunk configuration.

All SIP codesFind a fault by symptom