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ATIS/SIP Forum Innovation Delivers Robocall Call Blocking Notification Standard

New standard is another important new tool developed through ATIS and SIP Forum collaboration to enhance the industry’s means for blocking calls intended to defraud consumers

WASHINGTON and NORTH ANDOVER, Mass. (Aug. 25, 2022) — The SIP Forum and ATIS today announced the release of a new standard advancing the means for mitigating unwanted robocalling by providing a bounce back message to originating call parties to let them know their call has been blocked.

This message is both critical and strategic. If legitimate calls are improperly blocked, the caller needs to know this so that they can initiate actions to resolve the problem, including the contact information where legitimate callers can seek redress. For malicious parties that should be blocked, however, it is important that the feedback mechanism not provide information that might help aid in the development of strategies for unwanted calls to fool call-blocking analytics.

The Robocall Call Blocking Notification specifically addresses the needs of enterprises and others for timely, machine-readable and actionable notification when a call from a legitimate calling party is blocked by the indicated voice service provider due to analytics-based call processing. This ensures that voice service providers can continue to use analytics to block calls suspected to be illegal, fraudulent or for other reasons undesirable, while providing real-time notice to callers. To provide such notification, this standard defines a profile of the SIP 603 response code defined in RFC 3261, SIP: Session Initiation Protocol referred to in the standard as “603+”.

“This new standard is another important new tool developed through ATIS and SIP Forum collaboration to enhance the industry’s means for blocking calls intended to defraud consumers,” said ATIS President and CEO Susan Miller. “It balances the need to provide legitimate callers with a mechanism to seek redress if they have been inappropriately blocked against the desire to limit feedback to malicious callers.”

“We join with ATIS and others in encouraging widespread adoption of this specification across the entire call processing ecosystem with ‘all deliberate speed,’” said SIP Forum Chairman of the Board Richard Shockey. “Vendors and service providers will need some time to develop code and test, but those of us in the technical community are convinced that 603+ is the best, shortest and least-costly path to implementation and success. The SIP Forum deeply values its cooperation with ATIS in our joint Network to Network Task Force that continues to advance the technical state of the art. The work continues.”

Access the Robocall Call Blocking Notification standard document here.

About the SIP Forum

The SIP Forum is a leading IP communications industry association that engages in numerous activities that promote and advance SIP-based technology, such as the development of industry recommendations; interoperability testing events including SIPit, and special workshops, educational activities, and general promotion of IP communications standards, services, and technology for service provider, enterprise and governmental applications. The SIP Forum is also the producer of the annual SIPNOC conferences (for SIP Network Operators Conference), focused on the technical requirements of the service provider community, the STIR/SHAKEN VIRTUAL SUMMIT, and other industry events. Other important Forum initiatives include maintaining working groups in SIP trunking, security, SIP and IPv6, and IP-based Network-to-Network Interconnection (IP-NNI) and the SHAKEN Call Authentication Framework. For more information about SIP Forum initiatives, please visit:

About ATIS

As a leading technology and solutions development organization, ATIS brings together the top global ICT companies to advance the industry’s business priorities. Our Next G Alliance is building the foundation for North American leadership in 6G and beyond. ATIS’ 150 member companies are also currently working to address 5G, robocall mitigation, IoT, Smart Cities, artificial intelligence-enabled networks, distributed ledger/blockchain technology, cybersecurity, emergency services, quality of service, billing support, operations and more. These priorities follow a fast-track development lifecycle – from design and innovation through standards, specifications, requirements, business use cases, software toolkits, open source solutions and interoperability testing.

ATIS is accredited by the American National Standards Institute (ANSI). ATIS is the North American Organizational Partner for the 3rd Generation Partnership Project (3GPP), a founding Partner of the oneM2M global initiative, a member of the International Telecommunication Union (ITU), as well as a member of the Inter-American Telecommunication Commission (CITEL). For more information, visit Follow ATIS on Twitter and on LinkedIn.


SIP Forum Contact

Marc Robins

President & Managing Director

+1 203-829-6307

ATIS Contact

Marcella Wolfe

Manager, Public Relations & Marketing Communications

+1 202-434-8851

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