Azure Comms, Declined CODE: 603 issue...

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Azure Comms, Declined CODE: 603 issue...

Dylan Olney 100

Hi,

I've developed an application for a client that uses the Azure Comms Calling service. It has been working fine for the most part, but just recently users are reporting that some outgoing calls aren't being connected. The reason given by the call agent is: Declined CODE: 603. However, we know that the recipients aren't declining these calls themselves, as our testing has shown that when this happens, the recipient's phone doesn't even ring.

I Googled the issue and the AI pulled the information shown below from Microsoft.

I have crossed out the lines below which we have established as not being the cause, nor relevant, which leaves us only with the possibility that the recipient's carrier is not allowing the call through for whatever reason. Is it possible that our Azure number may have been blacklisted as spam by a carrier? We know from tests that at least calls to the Three Ireland carrier calls are failing (+353 86). If our Azure number has been blacklisted, what action can we take to un-blacklist it and ensure it doesn't get blacklisted again?

This is a breaking issue in our application which has been in production and in daily use by a company for over a year, so any help with this would be greatly appreciated,

Yours Truly,

Dylan Olney.

Meaning of 603 Call Declined:

Definition: The person or system you were trying to call was reached successfully, but they explicitly declined or rejected the call.

Context: This is often considered "expected behavior" in SIP/telephony scenarios, representing a user action rather than a system failure.

PSTN Context: For outbound PSTN (phone) calls, this can sometimes indicate the call was blocked by the carrier, flagged as spam, or the user is not entitled to call that specific destination.

Troubleshooting and Solutions:

Check User Actions: Verify if the recipient is intentionally declining the call.

Verify Phone Numbers: If this happens on outbound calls to phone numbers, ensure the number is valid, active, and properly formatted in E.164 format.

Carrier/Policy Restrictions: If many calls are failing, it could be a fraud/abuse policy check on the carrier side, particularly if the destination is blocked.

Try Alternative Numbers: Test if the issue persists with different numbers or if it is restricted to a specific destination/pattern.

Check CallAutomation/SDK Logs: Analyze the specific ParticipantEndReason subcodes in your logs to get more detailed information.

Dylan Olney 100 Reputation points

2026-04-21T16:04:15.87+00:00

^^^ Update: Whether the outgoing call connects does seem to be carrier related. We tested with a call to a different carrier and that got through. Like I said, the Three Ireland carrier is at least one that we know of that's blocking outbound calls all of a sudden. I suppose we ought to contact that carrier to see if they can do anything about it. The query remains open though...
Shree Hima Bindu Maganti 7,420 Reputation points Microsoft External Staff Moderator

2026-04-23T17:36:26.4366667+00:00

hi @Dylan Olney
Thank you for the detailed update. Based on the symptoms you described, where the destination phone does not ring and the issue is affecting specific mobile carriers such as Three Ireland, this indicates the call is most likely being rejected by the downstream carrier network rather than by Azure Communication Services itself. In SIP telephony, a 603 Decline response means the call was declined somewhere in the call path, and in PSTN scenarios that response can be returned by the terminating carrier before the call reaches the recipient handset. Since calls to other carriers are succeeding, this further supports that the behavior is carrier-specific and not a general Azure service failure. Microsoft’s official troubleshooting guidance explains that call end reasons and SIP response codes should be reviewed to identify whether the rejection came from the carrier or another part of the route.
https://learn.microsoft.com/en-us/azure/communication-services/resources/troubleshooting/voice-video-calling/troubleshooting-codes?pivots=automation
It is possible for numbers to be filtered or flagged by carrier spam prevention systems based on traffic patterns, call volume, or reputation scoring. Azure does not directly control third-party carrier spam blocks, so there is no self-service option to “un-blacklist” a number from the Azure side. The recommended next step is to open a Microsoft Azure Support request with several failed call examples including UTC timestamps, source Azure phone number, destination numbers, and Call Automation/Calling correlation IDs so the backend team can trace the PSTN route and engage telecom partners if needed. You may also contact Three Ireland directly, as they control final delivery to their subscribers. For future prevention, Microsoft recommends following standard calling best practices such as using valid caller ID, avoiding aggressive burst dialing, and maintaining compliant outbound calling behavior
https://learn.microsoft.com/en-us/azure/communication-services/support
Let me know if you have any further assistance needed.
Dylan Olney 100 Reputation points

2026-04-24T10:01:32.4133333+00:00

Thanks @Shree Hima Bindu Maganti ,

The issue seems to have been resolved all by itself. Outgoing calls to numbers on the Three Ireland network are now working for us again. Whatever the reason was, it seems to have been temporary, lasting only ~ 24 hours. Here's hoping it doesn't happen again...

Thanks again,

Dylan.
Shree Hima Bindu Maganti 7,420 Reputation points Microsoft External Staff Moderator

2026-04-27T16:29:53.4333333+00:00

hi @Dylan Olney
If the answer is helpful, Please do click "Accept the answer” and Yes, this can be beneficial to other community members.

If you have any other questions, let me know in the "comments" and I would be happy to help you

Answer accepted by question author

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Dylan Olney 100 Reputation points

2026-04-21T16:04:15.87+00:00

^^^ Update: Whether the outgoing call connects does seem to be carrier related. We tested with a call to a different carrier and that got through. Like I said, the Three Ireland carrier is at least one that we know of that's blocking outbound calls all of a sudden. I suppose we ought to contact that carrier to see if they can do anything about it. The query remains open though...
Dylan Olney 100 Reputation points

2026-04-24T10:01:32.4133333+00:00

Thanks @Shree Hima Bindu Maganti ,

The issue seems to have been resolved all by itself. Outgoing calls to numbers on the Three Ireland network are now working for us again. Whatever the reason was, it seems to have been temporary, lasting only ~ 24 hours. Here's hoping it doesn't happen again...

Thanks again,

Dylan.
Shree Hima Bindu Maganti 7,420 Reputation points Microsoft External Staff Moderator

2026-04-27T16:29:53.4333333+00:00

hi @Dylan Olney
If the answer is helpful, Please do click "Accept the answer” and Yes, this can be beneficial to other community members.

If you have any other questions, let me know in the "comments" and I would be happy to help you

Answer 1

hi @Dylan Olney
Thank you for the update. I’m glad to hear that outgoing calls to the Three Ireland network are now working again. Since the issue resolved without any configuration changes on your side, it was most likely a temporary routing, interconnect, or carrier-side filtering condition in the downstream PSTN path. These kinds of issues can occasionally clear automatically once the carrier refreshes routing tables or removes temporary network restrictions.

If the behavior happens again, we recommend capturing a few failed call examples with the UTC timestamp, source Azure Communication Services number, destination number, and any call correlation IDs so that a support investigation can trace the call route more quickly. You can also continue monitoring call diagnostics through Azure Communication Services logs for any recurring patterns.
https://learn.microsoft.com/en-us/azure/communication-services/resources/troubleshooting/voice-video-calling/troubleshooting-codes?pivots=automation
If the answer is helpful, Please do click "Accept the answer” and Yes, this can be beneficial to other community members.

If you have any other questions, let me know in the "comments" and I would be happy to help you

Answer 2

For Azure Communication Services PSTN calls, a 603 “Declined” can be returned in multiple scenarios, not only when the human callee presses “decline.” In ACS, 603 is documented as a successful end state from the platform’s perspective when the call is rejected anywhere along the path:

Code 603 with ParticipantEndReason means the call was rejected without connecting.
For PSTN specifically, the documented causes include:
- Call declined by the recipient.
- Call declined due to fraud detection.

The guidance for PSTN 603 is:

“Ensure that you aren't exceeding the maximum number of concurrent calls allowed for your Azure Communication Services phone number. For more information, see PSTN call limitations.”

In other words, for PSTN calls a 603 can be returned when:

The recipient or their device rejects the call, or
A carrier or fraud-detection system in the path rejects/blocks the call (for example, spam/fraud detection), or
Platform limitations/policies (such as PSTN call limitations) cause the call to be rejected.

The documentation in the provided context does not describe a direct, self-service way to “un‑blacklist” or reset a number that a carrier has flagged as spam. It only indicates that 603 may be due to fraud detection and points to checking PSTN call limitations and concurrent call usage.

Given that:

Calls to specific carriers (such as Three Ireland) are failing with 603 without the phone ringing, and
603 for PSTN can be due to fraud detection or policy restrictions,

the supported actions from the documentation are:

Confirm that the calls are within ACS PSTN call limitations and that the maximum concurrent call limits for the ACS phone number are not being exceeded.
Treat 603 as a rejection at some point in the call path (recipient, carrier, or fraud-detection system). If the issue persists and is impacting production, open a support request with Microsoft (Azure support) so that Microsoft and the underlying carriers can investigate whether fraud/spam policies are affecting the ACS number and what remediation is possible.

The context does not provide additional technical steps or APIs to directly remove a spam/blacklist flag at the carrier level; escalation through Microsoft support is required for deeper investigation and remediation.

References:

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Azure Comms, Declined CODE: 603 issue...

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