Error reference

SIP 503 Service Unavailable

Short answer

A 503 from Cloud SIP usually says Channel Limit Reached, which means the call would go over your channels. If the Console shows NO_CREDIT, the account has no credit available for calls. If only calls to one number or network fail, the 503 came from the phone network beyond Enfonica.

Applies to
Cloud SIP, Teams Direct Routing
Works with
Any SIP PBX or SBC, Microsoft Teams Direct Routing
Who can do this
Whoever manages your PBX and your Enfonica project

Check for a known incident first. If there is one, we're already working on it, and there's nothing to change on your side.

Checking Enfonica system status…

Status page
On this page
  1. What are you seeing?
  2. Calls fail with 503 Channel Limit Reached
  3. Every outgoing call fails with 503, and the Console shows NO_CREDIT
  4. Calls to one number or network fail with 503
  5. A few calls fail with 503 for a short time
  6. Calls to a Teams user fail with 503
  7. How do I see the SIP response?
  8. Questions
  9. What to send us
  10. Related

What are you seeing?

Calls fail with 503 Channel Limit Reached

SIP/2.0 503 Channel Limit Reached

Cloud SIP refused the call because it would go over a channel limit. A channel is one outgoing call in progress, so the limit is how many outgoing calls can be up at once. The response also carries Reason: Q.850;cause=44;text="Channel Limit Reached".

Your calls would go over a channel limit

Check

Look at when calls fail. This is the cause if outgoing calls fail only while other calls are in progress, and a single call on its own goes through. Calls coming in to you keep working. Cloud SIP refuses these calls before they start, so they appear in your PBX's log but not in Cloud Voice > Calls.

Fix

Raise the limit. Where you raise it depends on your plan:

  • Bundle plan or Teams Direct Routing SIP domain. The limit is set on the SIP domain in Cloud SIP, under Bundle plan or Channel limit.
  • Pay-as-you-go. The limit is your account's limit on concurrent calls, which we can raise on request.

For the steps on each plan, and what counts toward the limit, see Some calls fail when you're busy.

Every outgoing call fails with 503, and the Console shows NO_CREDIT

NO_CREDIT

Before each outgoing call, Cloud SIP checks that the account can pay for it. If the billing account has no credit available, or billing isn't active for the project, Cloud SIP refuses the call with a 503.

The account has no credit available

Check

Open the failed call in Cloud Voice > Calls. This is the cause if it shows the error code NO_CREDIT. Every outgoing call fails, whatever number you dial, and calls to your numbers fail too.

Fix

See Why have my services stopped? It shows how to check whether the account has reached its credit limit, run out of prepaid credit, or has an overdue balance. It also shows how to restore service.

Calls to one number or network fail with 503

SIP/2.0 503 Service Unavailable

If only calls to one number, number range, or network fail, while other calls go through, your channels and your account aren't the cause. The 503 came from the phone network beyond Enfonica that the call was passed to, and we pass it back to your PBX unchanged.

The network beyond Enfonica couldn't complete the call

Check

Look at the response and the failed calls. This is the cause if the response doesn't say Channel Limit Reached, your other calls work, and the failures share a destination. Some networks also answer calls to a disconnected number this way, often after ringing or progress tones for about 20 seconds.

Fix

Call the number from a mobile to check that it's in service. If it works from a mobile, send us a few example calls with the details listed below. We'll trace them and take it up with the network responsible.

A few calls fail with 503 for a short time

Reason: Q.850;cause=38

Before maintenance, part of Enfonica's edge stops accepting new calls, and answers them with a 503 and this Reason header. The rest of the edge keeps working. Calls already in progress aren't affected.

The call reached part of the edge that was going into maintenance

Check

Look at the 503 and try the call again. This is the cause if the 503 carries Reason: Q.850;cause=38, and the same call made again a moment later works.

Fix

Check the status page, and then set up your PBX so that calls keep working during maintenance:

  • Point your PBX at your SIP domain's name instead of an IP address. The name resolves to both signaling addresses, 34.87.239.197 and 34.87.254.143.
  • Allow both addresses in your firewall.
  • If your PBX can try another address after a 503, turn that on.

Calls to a Teams user fail with 503

Reason: Q.850;cause=34

On incoming calls, Microsoft Teams answers a call to a user who's already on a call with a 503 and this Reason header. Enfonica treats it as busy, so the caller hears a busy tone, and the call isn't retried.

The Teams user is busy

Fix

Ask your IT team or MSP to change the setting in your Microsoft 365 tenant. They can give the user voicemail, or route their calls through a call queue, so that busy calls go somewhere. For the options, see Teams Direct Routing calls fail or don't ring.

How do I see the SIP response?

Most PBXs show the SIP response for a failed call in their call log or a SIP trace. Look for the line that starts with SIP/2.0 503, and the Reason header in the same response. The words after the code, and the cause number in the Reason header, tell the causes on this page apart.

Calls that Cloud SIP accepted also have a SIP trace in Cloud Voice > Calls. To learn how to read it, see Find a call and read its SIP trace.

For other errors on outbound calls, see Outbound calls aren't going through.

Questions

Does my account have other limits on calls?

Yes. Each account has limits on concurrent calls and on outgoing call attempts per second. For the limits, see Cloud SIP limits. Cloud SIP scales with you, and these limits can be increased on request or during onboarding.

What to send us

Include these and we can find the call straight away.

  • The number that made the call and the number it called, in full international format (for example +61255501234).
  • The date and time of the call, with your timezone.
  • Whether the call was inbound or outbound.
  • The call ID, if the Console shows the call.
  • What happened: an error message, silence, a busy tone or a dropped call.
Contact support

Our support team are telco specialists, backed by the engineers who run the network.