[VoiceOps] Caller-ID: SIP From vs. P-Asserted-Identity

Jay Hennigan jay at west.net
Tue Jul 24 00:50:21 EDT 2012

On 7/23/12 5:32 PM, Jeff MacDonald wrote:
> Greetings,
> On Monday, July 23, 2012 07:58:29 PM Colin Brown wrote:
>> The best way is to define and agree with each termination carrier how they
>> will handle it. I'm speaking about mostly sending international traffic
>> and believe  me they all want it different ways.
> Does the PBX software easily accomodate something like that?

The situation comes up most often when migrating a customer from a
TDM-based PRI to a PRI delivered over SIP to an IAD.  Also in the call
center/agent scenario described earlier.

In the PRI/TDM world, the PBX is quite capable of sending the inbound
CLID on a hairpinned call, and the RBOC is happy to deliver whatever is

We want to use PAI to keep our CDRs straight, ensure that 9-1-1 routes
correctly, handle subpoenas easily, etc.

We have two primary domestic termination carriers.  One takes the
information in the From: for display on CLID, the other uses PAI.

RFC3325 section 7 refers to inserting an "id" token to the privacy
header field to preclude forwarding PAI to an untrusted party (which I
would think includes an end user's CLID display).  Whether this would
result in the From: being used or just a "Private" display, I don't know
and I haven't found the appropriate knob on any of our gear yet to test

Section 8 of the same RFC seems to indicate that how or if PAI is passed
on is somewhat of a crapshoot.  My inquiry to the list was to see if any
form of consensus has been reached in the decade since this RFC was

Summary so far of on-list and off-list replies:

1. Use a diversion header.
   *  Not recommended if call was not actually diverted.
   *  Legacy PBX via PRI doesn't tell if diverted or not.

2. Carriers generally prioritize PAI then RPID then From for CLID.

3. Contact the carrier and ask them to use From.  Reasonable carriers
should be able to do this on a per-trunk-group basis.

4. Sending both PAI and RPID is bad, especially if they are different.

We are pursuing option 3.

Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV

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