Calling issue

Hi @Satram Likhitha,

Thank you for describing the issue in detail.

When outbound PSTN calls from Microsoft Teams are dropping and you see SIP response codes 504 and 487 in the Teams admin center, this usually points to a connectivity or signaling problem during call setup to external phone numbers, even though the user has a Domestic Calling Plan assigned.

To explain a bit more clearly, SIP 504 (Server Time‑out) typically occurs when Microsoft Teams is unable to successfully deliver the INVITE request to the PSTN connectivity provider. This can be caused by network‑level issues such as TLS, DNS resolution problems, firewall or proxy interference, or temporary service-side delays. In many cases, the follow‑up SIP 487 (Request Terminated) appears because the call was cancelled early, often as a result of the timeout, an aborted signaling attempt, or a downstream media negotiation failure triggered by the initial 504 response.

As a first step, I recommend verifying the basic voice configuration for the affected user. Please confirm that the correct Domestic Calling Plan license is assigned, a valid phone number is properly provisioned, and the PSTN usage location is set correctly.

Next, it’s important to review the network path. Please check whether any firewalls, proxies, or security devices might be blocking or inspecting Teams PSTN traffic. It’s also helpful to review the PSTN Usage in the Teams admin center to look at detailed Call IDs and Final Microsoft subcode values. These details can provide additional insight into where the call is failing and help indicate the appropriate corrective actions you can take.

If the issue persists, or if you would like the problem to be thoroughly investigated and resolved accurately, the best next step is to open a support ticket with Microsoft so the case can be reviewed properly. You can follow the guidance here: Get support - Microsoft 365 admin | Microsoft Learn (If you're an end user, please contact your Microsoft 365 administrator and ask them to submit a support request on your behalf)

Once the ticket is opened, a Microsoft technical support engineer can review backend configurations, analyze signaling logs, and, if needed, run synchronization or diagnostic tools that aren’t available from the admin portal. Should the investigation require deeper analysis, the case can also be escalated to a specialized engineering team.

Please keep in mind that this is a user‑to‑user support forum. Moderators here are regular community users and don’t have the permissions or tools to directly access tenant data or fix issues at the service level. Guiding you toward the most effective troubleshooting steps and the correct support path is the best assistance I can provide within this space.

I hope your issue gets resolved soon. If you have any updates, please feel free to reply and share them. I’ll be happy to continue the discussion and help where I can.

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For PSTN calls from Microsoft Teams that drop with SIP 504 and 487 codes in the Teams admin center, focus on two areas: Direct Routing/SBC connectivity and general infrastructure issues.

1. Interpret the codes

• 504 in this context generally indicates a timeout or infrastructure problem on the path to or from the Session Border Controller (SBC) or PSTN provider, or a Microsoft infrastructure error.
• 487 typically indicates the call was canceled (for example, by the caller or due to a timeout condition being hit before the call could be fully established).

2. If using Direct Routing (SBC involved) Check the detailed Microsoft CallEndSubReason (six‑digit Microsoft response code) that appears together with 504 in the Teams admin center. Then apply the corresponding action:

• If the subreason indicates TLS transport not connected or connection refused (for example, 549002/569002 with 504):
  • Verify the SBC is up and functional.
  • Confirm the SBC is reachable on the configured IP and port from Microsoft (check firewall rules and any intermediate network devices to ensure traffic to/from the SBC is not blocked).
• If the subreason indicates TLS negotiation failure or certificate issues (for example, 569003, 569008, 569009 with 504):
  • Ensure the SBC certificate is not expired and is signed by a trusted public CA supported for Direct Routing.
  • If the certificate is expired (569008), renew the SBC certificate and then restart the SBC or otherwise force closure of existing TLS sessions so new sessions use the renewed certificate.
  • If the error indicates untrusted root or wrong usage (569009, CERT_E_WRONG_USAGE), install a certificate from a supported public root CA and ensure the correct TLS profile/certificate is used for the Direct Routing interface.
• If the subreason indicates DNS issues (569015 or 569016 with 504):
  • Verify that the SBC FQDN resolves correctly in DNS.
  • Ensure that appropriate A/AAAA records exist for the SBC FQDN (not just TXT records used for domain verification).
• If the subreason indicates invalid SIP Request/Response line (549002, 549018, 569018 with 504):
  • Review the SBC configuration and SIP traces to ensure SIP messages conform to SIP/2.0 and are in a format supported by Microsoft.
  • Correct any misconfiguration that causes malformed or unsupported SIP messages.

3. If using Calling Plans / Azure Communication Services

• For Azure Communication Services, 500/503/504 codes are categorized as infrastructure errors. The guidance is to file a support request through the Azure portal so Microsoft can investigate backend issues.

4. Confirm user configuration

• Since the user has a Domestic Calling feature, verify in the Teams admin center that the user is correctly configured to make PSTN calls (license assigned, phone number and emergency address assigned as required for Calling Plans). Use the available self-diagnostics in the Microsoft 365 admin center to validate PSTN configuration if applicable.

5. Next steps

• Pull the exact Microsoft CallEndSubReason code associated with the 504 for the failing calls from the Teams admin center.
• Match that code to the documented 504 scenarios and apply the specific remediation (SBC reachability, certificate renewal/trust, DNS fix, or SIP formatting correction).
• If all SBC and DNS checks pass and errors persist, open a support ticket (Microsoft 365 or Azure, depending on whether Calling Plans or ACS/Direct Routing is used) and provide the SIP and media logs plus the CallEndReason/CallEndSubReason values.

References:

• SIP response code 504
• Microsoft and SIP response codes
• Set up Microsoft Calling Plans
• Understanding calling codes and subcodes errors (callend)