Elastic SIP Trunking - Pass on 603+ Reason Header Information

What’s changing?

Starting 23rd April 2026, Twilio will start passing on the “Reason” header for Elastic SIP Trunking outbound calls (Trunking Termination calls) that are rejected by Carriers based on  Network Analytics with a SIP error response 603 Network Blocked.

What does this mean for you?

• Twilio Elastic SIP Trunking customers will be able to see the Reason header(if provided by the carrier) within 603 Network Blocked SIP error responses for their Elastic SIP Trunking outbound calls.

• The Reason header (if provided by the carriers) may contain information about why the call was rejected or blocked.

• The Reason header may also contain information about how to appeal the call blocking. 

Example Reason headers: 

Reason:SIP;cause=603;text="v=analytics1;url=https://www.t-mobile.com/responsibility/consumer-info/additional-info/call-blocking-redress";location=RLN

Reason:SIP;cause=603;text="v=analytics1;url=https://www.att.com/redressC;id=att2";location=RLN

What action do you need to take?

• No additional Twilio configuration changes are required.

• The Reason header can be seen from the SIP messages by downloading the PCAPs from Twilio console call logs.

Need help or have questions?

Please refer to our Elastic SIP Trunking documentation or contact Twilio Support for guidance.