SIP 603+ blocking notification standard approved

The SIP 603+ standard on robocall blocking notification has been approved and published by ATIS. Here’s an overview.

SIP 603+ blocking notification

The SIP 603+ response is used when a call is blocked due to analytics-based call processing. The SIP 603+ message is based on the standard SIP 603 message, with two differences:

  1. The SIP 603+ status line uses a unique reason phrase, “Network Blocked,” rather than the SIP 603 default reason phrase “Decline.”
  2. The SIP 603+ contains a SIP Reason header, which is a standard feature of SIP messaging. The ATIS-1000099 standard describes specific encoding that is put into this Reason header for a SIP 603+ message.

Reason header encoding

The SIP 603+ reason header includes three parameters listed in table 1:

Table 1. Reason header parameters

“cause”Yes“21” or “603”
“text”YesSee table 2
“location”Yes“LN”, “TN”, “LPN”, “RPN”, or “RLN”

The location codes are defined in another standard and are leveraged for this standard. These codes indicate where in the network the block occurred:

  • LN: Originating network
  • TN: Transit network
  • LPN: Originating private network
  • RPN: Private network serving the called party
  • RLN: Network serving the called party

The “text” parameter is a string of attribute-value pairs that can be used to relay several different pieces of information listed in table 2:

Table 2. “text” parameter attribute-value pairs

“url”if neither “tel” nor “email” are includedValid HTTPS URL for the calling party to visit for redress
“tel”if neither “url” nor “email” are includedValid E.164 formatted telephone number for the calling party to call for redress
“email”if neither “url” nor “tel” are includedValid email address for the calling party to email for redress
“id”NoIdentifier used by the service provider that blocked the call to facilitate redress (e.g., call identifier, blocking reason identifier, network segment identifier, etc.

Reason header examples

Reason: SIP; cause=603; text="v=analytics1;url=";location=LN

This call was blocked at the originating network. The caller can visit website for redress.

Reason: Q.850; cause=21; text="v=analytics1;tel=+12155551212";location=RLN 

This call was blocked at the network serving the called party. The caller can call (215) 555-1212 for redress.

Reason: SIP; cause=603; text="v=analytics1;url=;;tel=+12155551212;id=29016905-3bed4c98-9423-03041160cc67";location=LN

This call was blocked at the originating network. The caller can either visit the webpage, send an email to, or call (215) 555-1212 for redress. A call ID number was provided to facilitate redress.

How we got here

In December 2019, the TRACED Act was signed into law. It amended the U.S. Code to require the FCC to act within one year to ensure that robocall blocking services are provided with transparency and effective redress options for both consumers and callers.

The FCC responded in December 2020 with their Fourth Report and Order on robocall mitigation. This required immediate notification of blocking be implemented by January 1, 2022, using SIP code 607 or 608 for calls on IP networks and ISUP code 21 for calls on TDM networks.

In May 2021, USTelecom filed a petition for reconsideration with the FCC for the Fourth Order’s notification requirement. Instead, the Commission should give service providers flexibility in how to notify callers that their calls have been blocked. They said that the standards were not finalized, and service providers would not be ready by the deadline. If blocking required such notification, then providers would have to stop blocking robocalls.

In December 2021, the FCC issued an Order on Reconsideration that allowed service providers to use SIP code 603 as a temporary measure to meet the immediate blocking notification requirement. The Commission issued a Further Notice of Proposed Rulemaking, in which they asked if SIP 603+ would provide sufficient notification or should SIP 607/608 eventually be required. Nine organizations replied:

  • Commenters representing callers and some industry associations wrote that SIP 603 does not provide actionable information for redress. Therefore, it doesn’t satisfy the TRACED Act.
  • Commenters representing carriers and an analytics firm wrote that SIP 603 is adequate, or at least the most pragmatic choice. They also said there’s a better SIP 603+ under discussion.

From March through July 2022, standards development continued on SIP 603+. As work progressed, members of the standards task force reached out to associations representing high volume callers for feedback on whether the SIP 603+ specification would meet their needs. The calling community responded with a list of their requirements. By our tally, it seemed the answer would be “yes.”

In August, the ATIS IP-NNI Forum approved and published the ATIS-1000099 standards document.


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