Call blocking notification details emerge for SIP 603+

The TRACED Act requires immediate call blocking notification. An updated version of the SIP decline message, dubbed “SIP 603+” has been proposed. Would this be sufficient for notification and redress? In this article, we summarize the proposal so you can decide.

These are the first examples of SIP 603+ that we’ve seen. The design may change with further discussion.

SIP 603+ example

Let’s cut to the chase with an example. Instead of "Decline," the reason phrase for the SIP 603+ message is "Network Blocked."

A Reason Header is added to the SIP 603+. Here’s an example:

Reason: Q.850;cause=21;text="v=analytics1;";id=e0c1913c-b39c-4ca4-bcaa-20c9d60e4e26;location=LN

Let’s highlight sections of this example to help explain:

Reason: Q.850;cause=21;text="v=analytics1;";id=e0c1913c-b39c-4ca4-bcaa-20c9d60e4e26;location=LN
  1. The reason key, related parameters and location key in yellow are required.
  2. The contact information and location code in blue require something, but there are options for what can go there.
  3. The blocking event identifier in green is optional.
businesswoman talking on phone with headset

1. Reason key and location key

The first part in yellow enables network software to identify this information as a call blocking notification. The cause code 21 would be used by a SIP/TDM media gateway.

The location parameter at the end of this example identifies where the call was blocked along the call path. Options for the location parameter value include:

  1. LN — originating network
  2. TN — transit network
  3. LPN — originating private network
  4. RPN — private network serving the called party
  5. RLN — network serving the called party
SIP 603+ blocking location codes

Fig 1. SIP 603+ Blocking Location Codes—Where Was The Call Blocked?

2. Blocking contact and location type

The call blocker must identify themselves in the SIP 603+ with at least one of the following:

  1. Email address, such as “”
  2. Website address, such as “url=”
  3. Telephone number, such as “tel=+14045551212”

The call blocker may provide more than one of these contact types if they wish.

3. Blocking event identifier

The call blocker may include an identification code if they wish. This would help them track down the call blocking event if the block is challenged by a caller with a redress request.

Would SIP 603+ be sufficient?

SIP 603+ is clearly a big improvement over a plain old SIP 603. Would it be sufficient to satisfy the redress requirements in the TRACED Act?

We look forward to hearing comments from those who have expressed concern over the use of SIP 603 and have been advocating for SIP 607/608 to satisfy the blocking notification requirement.

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