Call blocking notification discussion at SIPNOC

There’s been an ongoing debate for almost a year now about how to satisfy the immediate call blocking notification requirements of the TRACED Act. The conversation continued with a very interesting session at SIPNOC 2022. Here are the highlights.

Panel discussion

The panel discussion was titled Resolving the 603/607/608 Call Blocking Notification Codes Issue. The panelists were:

  • Richard Shockey, SIP Forum Chairman and moderator of this discussion
  • Eric Burger, Research Professor of Computer Science, Georgetown University, and coauthor of the SIP 608 IETF specification
  • Phil LInse, Director of Public Policy at Lumen
  • Timothy Dwight, Fellow at Verizon Technology


  1. December 29, 2020. The FCC mandated immediate notification for call blocking in their Fourth Report and Order on robocall mitigation. The order required the use of SIP 607 and SIP 608, new SIP messages. The implementation deadline was January 1, 2022.
  2. May 2021. USTelecom filed a petition for reconsideration saying that carriers wouldn’t be ready and would have to stop blocking robocalls.
  3. December 10, 2021. The FCC issued an Order on Reconsideration, which said carriers could use SIP 603 temporarily, and a Further Notice of Proposed Rulemaking, which proposed that SIP 607/608 was the best long-term solution and asked for comments.
  4. Since then, USTelecom and others began advocating for an enhanced blocking notification dubbed SIP 603+. It’s a SIP 603 message with additional information intended to satisfy the immediate notification requirements.


Here’s a summary of points made during this discussion advocating for and against SIP 603+ and SIP 608.

Phil Linse

Phil said that network equipment and software in use are already capable of handling the SIP 603+. This is a regular SIP 603 with standardized messages placed in a reason header text field. These standardized messages explain who blocked the call and how to contact them.

In contrast, SIP 608 support would have to be added to network equipment and software.

SIP 603+ support would begin to appear in the network sooner. Some carriers are doing this now, although not in a standardized way, as the standards are still under development.

SIP 608, however, would take longer to support.

Tim Dwight

Tim said that both 603+ and 608 provide the necessary information that callers need for redress. But with 603+, we can get a solution into our networks faster. Network equipment doesn’t require modification or requires less modification than SIP 608.

There are two difficulties with 608:

  1. PKI infrastructure
  2. Audio message played after decrypting jCard and transcribe text to speech to generate an audio message.

Carriers would have to buy lots of infrastructure to make this happen.

Eric Burger

Could SIP 603+ signal to the carrier what the FCC requires? Absolutely.

There’s an assumption that, since I’ve got boxes that spit out 603s, that’s gotta be easier than doing 608s, right? No. If you look at the code that must handle the new information, it’s identical whether the information is in a 603+ or 608.

What’s different is that most software handles the result codes in a case statement. The question is, do we put in one more case (for 608), or do I put a bunch of if-then-else statements within the 603 case?

The 608 spec was published in 2018, almost 4 years ago. The RFC was published over a year ago. If 608 is so hard, where were the comments back then? What’s different is that the FCC said, “no, really, you need a solution.”

More information

The entire video recording is available online. (Registration is required to view the recording.) It’s an interesting discussion and well worth a look.


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