[VoiceOps] STIR/SHAKEN warning!

[Nathan Anderson]

I guess I should clarify that what I intended to communicate by "a few years
ago" would be more, like, 5-10 years or even more; heh.  And I'd wager that
even for as rapidly as SIP has been adopted in the corporate world, even a
decade ago there had to be considerably more Fortune 500s out there with a wide
installed base of PRI trunks relative even to today.  And of course, if anybody
were using their TFN as their outbound CID more than 2-3 years ago, not only
would S/S not even have been much of a thing yet at that point (to the extent
it was implemented at all, my understanding was that it was mostly being
bootstrapped with self-signed certs), but apparently there was also no
authoritative CNAM database for TFNs prior to March 2020 as was just clarified
in this thread.  So it makes sense that now that both of those things exist +
how many inroads SIP trunking has made into enterprises that the use of TFNs in
CID has exploded in just a couple of years' time.

 

Just as a potentially interesting data point, we have had TFNs parked with
various SIP-first providers (who also act as RespOrgs as well), none of which
have yet to pass a single S/S PASSporT/Identity header our way on any calls
made to any of those numbers...

 

By the way, love your service.  Thanks for putting it together and maintaining
it...what a gift to the community.  There are a handful of S/S testing tools
out there, but yours is easily the most comprehensive...most just audibly read
back to you whether or not your call was attested, and if so at what level.  
Nothing about who signed it, whether the backing cert and entire chain of trust
is valid or not, and so on, as yours does.  And since most other tests only
provide one call-in number, if the call path between you and that testing tool
happens to go through an intermediate carrier that strips out our PASSporT and
either replaces it with their own or drops it entirely, tough noogies.  Whereas
you provide multiple call-in numbers across multiple carriers, which not only
provides opportunity to work around such issues, but gives you some idea of
call paths that might be breaking transmission of your PASSporT and which ones
are okay.  Super helpful!

 

From: David Frankel [mailto:dfrankel at zipdx.com]
Sent: Friday, July 7, 2023 3:21 PM
To: Nathan Anderson; 'Voice Ops'
Subject: RE: [VoiceOps] STIR/SHAKEN warning!

 

Nathan,

 

Regarding TFN’s as caller-ID: This became quite popular more than a mere few
years ago. Many customer support operations (banks, brokerages, airlines,
insurance companies, credit card companies) place outbound calls using their
toll-free customer support number as caller-ID. In our RRAPTOR robocall
surveillance platform, we have captured many, many thousands of calls with a
TFN as the calling number. And the majority of those calls are signed, with
varying levels of attestation. Most enterprise calls these days that I’m
familiar with are initiated over SIP trunks (not PRI or analog trunks or POTS
lines) and include the calling (FROM) number in the SIP INVITE.

 

Regarding signatures on calls TO TFNs: This also definitely happens. Our SHAKEN
identity test tool (at https://portal.legalcallsonly.org/Info/Identity) lets
users call any of several test numbers, including some TFNs. We see signatures
on many calls to those test TFNs. (We don’t save data for calls that do NOT
have signatures, so I can’t tell you the fraction.) I do know that there are
some toll-free providers that ALWAYS (or usually) have TDM in the call path.
Since IDENTITY headers don’t travel over TDM, those calls will not have
signatures at the terminating end.

 

David

 

From: VoiceOps <voiceops-bounces at voiceops.org> On Behalf Of Nathan Anderson via
VoiceOps
Sent: Friday, July 7, 2023 3:55 PM
To: Voice Ops <voiceops at voiceops.org>
Subject: Re: [VoiceOps] STIR/SHAKEN warning!

 

Thanks; I had no idea this was a thing.

 

-- Nathan

 

From: Paul Timmins [mailto:paul at timmins.net]
Sent: Friday, July 7, 2023 2:39 PM
To: Nathan Anderson
Cc: Voice Ops
Subject: Re: [VoiceOps] STIR/SHAKEN warning!

 

Always worth pointing out that in March 2020, Somos rolled out TFNIdentity. We
have it set up on customers who want to source from their TFNs, I haven't seen
many carriers actually look it up, but it does exist.

 

On Jul 7, 2023, at 5:34 PM, Nathan Anderson via VoiceOps <voiceops at voiceops.org
> wrote:

 

I suspect things might be different now (& I just haven't kept up), but
although it is clearly *possible* to transmit a TFN as the calling number /
CID, I seem to remember that even just a mere few years ago, it was HIGHLY
discouraged, and if you ever were to receive a call bearing a TFN as its CID,
it had a very high likelihood of being fraudulent or spam.  This was of course
back when the vast, vast majority of TFNs were essentially implemented as a
call forward or alias to a number that hung off of a local exchange.  So of
course outbound calls that many? most? companies with TFNs would make would
typically be sourced from their local exchange number(s) and not from the TFN
(s) (unless maybe a given company had a PRI and their provider allowed them to
source calls from their TFN?).  Thus the expectation for a long time (as I
understood it) was that TFNs were truly inbound-only and should be treated as
such.

 

Loosely tangentially related, as a purely anecdotal report, I will note that I
have yet to see a S/S signature/PASSporT attached to ANY calls made *to* ANY of
our TFNs, via any of the 3 SIP wholesalers we have used as both RespOrgs & for
actual traffic.

 

-- Nathan

 

From: VoiceOps [mailto:voiceops-bounces at voiceops.org] On Behalf Of David
Frankel via VoiceOps
Sent: Friday, July 7, 2023 7:52 AM
To: 'Ivan Kovacevic'; 'Voice Ops'
Subject: Re: [VoiceOps] STIR/SHAKEN warning!

 

Ivan asks: “How are you handling TFN atestations?”

 

When the signer of a call gives A-level attestation, it means that the signer
knows that the caller “is authorized to use” the calling number.

 

The signer can “know” that in any of a variety of ways. For toll-free numbers,
the most sophisticated and secure is probably via Delegate Certificates. SOMOS,
the North American Toll-Free Number Administrator, has commented about this in
a current FCC proceeding: https://www.fcc.gov/ecfs/document/10605623514445/1

 

As the signer, there are other ways you could determine that the caller is
authorized to use the number. For example, you could solicit some documentation
from them (like an invoice from their RespOrg and/or service provider) and you
could call the number and verify that your caller answers. The regulations
(today) do not specify exactly how you “know” so you (as the signer) need to
act in the spirit of the rules.

 

This problem is not unique to toll-free numbers. I might have a geographic
number that I obtain from provider A (and that’s how I get inbound calls to the
number), but I make outbound calls from that number via providers B and C for
redundancy and cost reasons.

 

Bear in mind that providers can set their own rules for what calls they will
accept and what attestations they will assign, and those rules can be more
restrictive than what might be dictated by regulation. For example, a provider
might say “I will only assign A-level attestation to calls that use calling
numbers assigned by me.” That’s their prerogative.  In fact, a provider might
say: “I will only accept calls that use calling numbers assigned by me. Those
calls will get A-level attestation. I will reject all other calls.” There are
no rules (to my knowledge) that prohibit providers from setting these kinds of
rules.

 

From: VoiceOps <voiceops-bounces at voiceops.org> On Behalf Of Ivan Kovacevic via
VoiceOps
Sent: Friday, July 7, 2023 7:27 AM
To: Voice Ops <voiceops at voiceops.org>
Subject: Re: [VoiceOps] STIR/SHAKEN warning!

 

Hopefully on-topic. How are you handling TFN atestations?  

 

Although a part of NANP - it's a different technology at the network level in
terms of chain of authority and routing.

 

RespOrg manages the number, but can provision and use many carriers to make
outbound calls using the TFN Caller ID (and to receive inbound calls via the
same TFN)... RespOrgs is not necessarily a carrier - who and how checks that
RespOrg has the authority in case of delegated attestation. I may be
overcomplicating it in my mind.. but it doesn't feel like the regulation maps
1-to-1 over to TFNs... Just wondering what everyone's experience is. 

 

Thanks,

 

Ivan

 

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