Twilio error 31203 is reported when a SIP REGISTER or INVITE request reaches Twilio's SIP infrastructure but Twilio cannot identify a valid account, SIP Domain, or Elastic SIP Trunk that matches the credentials or request URI in the SIP message. This error maps to a SIP 403 Forbidden or SIP 404 Not Found response sent back to the registering SIP device or calling system. It is almost always caused by a mismatch between the SIP domain or credentials configured in your SIP client and what exists in your Twilio account.
What Causes This Error
The most common cause is a SIP username or domain configured in your SIP client or softphone that does not match any SIP Domain configured in your Twilio account: the SIP Domain must exist under Console, then Voice, then SIP Domains and must be in the format yoursubdomain.sip.twilio.com or yoursubdomain.sip.us1.twilio.com depending on your account's region. Using a SIP credential (username and password pair) that has been deleted from the Twilio Console under the SIP Domain's Credentials section will cause Twilio to return a 403 because the credential no longer exists in the account. Pointing your SIP client at Twilio's Elastic SIP Trunking FQDN (pstn.twilio.com) using IP authentication but with an IP address that is not in the trunk's Origination IP Access Control List triggers a 31203 because the source IP does not match any authorized trunk. Sending a SIP REGISTER to the wrong Twilio region endpoint, for example registering to a US1 region endpoint from an account that was provisioned in the EU1 region, causes a 31203 because the account does not exist in the queried region.
How to Fix It Step by Step
Log into the Twilio Console and navigate to Voice, then SIP Domains to confirm that a SIP Domain exists with the exact subdomain your SIP client is targeting: if your client is configured to register to mycompany.sip.twilio.com, confirm that mycompany exists as a SIP Domain in the Console list. Click into the SIP Domain and verify the Credential Lists section shows the username and password your SIP client is using, adding the credential if it is missing by clicking Add a Credential List and creating the appropriate username and password pair. For Elastic SIP Trunking configurations, navigate to Elastic SIP Trunking, then your trunk, then Origination and confirm the IP address of your on-premise PBX or SIP gateway is listed in the Access Control List with the correct port and protocol. Test the SIP registration from your PBX or softphone and check the Twilio Debugger under Monitor, then Debugger for the SIP response code returned (403 vs 404 vs 407 Proxy Authentication Required), as each code points to a different configuration issue.
How to Prevent It from Recurring
Document all SIP Domain names, credential list entries, and Elastic SIP Trunk configurations in a team-accessible infrastructure registry, and require a change review process before any SIP domain or credential is modified in the Console, preventing accidental deletions that cause 31203 in production. Implement SIP registration monitoring using a lightweight SIP OPTIONS keepalive or REGISTER check from your PBX or monitoring system that fires every 60 seconds and alerts your team if a non-200 response is received, catching SIP authentication failures before they affect production calls. For Elastic SIP Trunking with IP authentication, automate the process of updating the ACL when your PBX's public IP changes (for example, using a dynamic DNS solution paired with a webhook that updates the Twilio Trunk ACL via the REST API when the IP changes). After any infrastructure change affecting SIP (PBX migration, credential rotation, domain rename), run a full SIP registration test from each configured endpoint before declaring the change complete.
When to Call a Specialist
If all Console configuration looks correct but 31203 errors persist, there may be a region mismatch that is not visible in the standard Console view: different Twilio account regions (US1, IE1, AU1, JP1) use different SIP endpoint FQDNs and credentials stored in separate data centers. A specialist can identify which region your account is provisioned in and confirm your SIP client is targeting the correct region-specific endpoint. You should also seek specialist help if you are migrating from one SIP Domain to another or consolidating multiple Twilio accounts and need to coordinate SIP credential migration without service interruption. Complex Elastic SIP Trunking configurations with multiple origination URIs, failover routing, and IP ACLs across multiple PBX sites are best validated by a specialist before going live to prevent production 31203 failures.
Conclusion
Error 31203 is a SIP account or credential lookup failure that is resolved by confirming your SIP Domain exists in the Console, your credentials are valid, and your SIP client is targeting the correct Twilio region endpoint. If this error is blocking your production system, contact our team and we will diagnose and fix it within the hour.
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