What is SIP 603+?
SIP 603+ is a response code based on the ATIS-1000099 standard that provides a specific, real-time per-call notification of analytics-based blocking (e.g., calls suspected of being illegal or fraudulent) across US carrier networks.
What's the difference between SIP 603 and SIP 603+?
While a standard SIP 603 response code simply indicates that a call has been declined, SIP 603+ identifies both analytics-based blocking and the party responsible for the block. It’s distinguished from a standard 603 response by two key elements:
A unique status line that uses the reason phrase "Network Blocked" instead of the default "Decline".
An enhanced Reason header that contains the reason for the blocking, versioning information, and identity of the blocking party.
What contact methods does the SIP 603+ Reason header include?
In addition to the identity of the blocking party, the SIP 603+ Reason header includes at least one of the following contact methods for the service provider that blocked the call:
URL: A link to a web page for the calling party to visit.
Telephone: A global E.164 formatted number.
Email: A valid support email address.
A standards-compliant SIP 603+ Reason header may appear as follows:
Reason:SIP;cause=603;text="v=analytics1;url=https://example.com;id=12345";location=LN
When does SIP 603+ go into effect?
The FCC has released an Order requiring service providers to begin issuing SIP 603+ response codes for analytics-based call blocking by March 25, 2026.
How is Bandwidth implementing SIP 603+?
Bandwidth will deploy the following three processing rule updates in tandem throughout March 2026 and apply them when forwarding received SIP 603+ responses to the originating calling party:
No code remapping: SIP 603 response codes won’t be changed to a different value.
No retries or route advancement: SIP 603 response codes won’t be subject to retries or route advancement.
Header integrity: SIP 603 response phrases and Reason headers will be passed transparently as long as they use valid syntax.
How do I reduce SIP 603+ responses?
Bandwidth offers Number Reputation Management (NRM), a comprehensive solution for addressing improper call blocking and labeling by analytics engines.
NRM can reduce improper blocking associated with SIP 603+ responses by proactively monitoring and remediating improper call blocking, helping protect brand trust and improve answer rates across all outbound calls. For more information, please contact your Bandwidth Account Manager.