It's not just T-Mobile, it was most US carriers.

Some examples:

- Vice paid a bounty hunter $300 to track a phone number [1]

- Police have paid these services to avoid warrant requirements, and corrections facilities use aggregator services to track numbers that inmates have calls with [2][3]

Apparently carriers claim to have stopped after getting fined $200m last year [4].

It was typically done through aggregators. EG, services that have similar access to multiple carriers and in turn expose a single endpoint to their own customers.

The aggregators pass on responsibility for obtaining consent to their end customers. Again, with no enforcement or ability for a target to opt out.

The only protection is an authentication requirement. But that just confirms you have a valid credential. Which you get either as an aggregator (to tmobile/other carrier directly), or as the client to an aggregator (to the aggregator's API to query multiple carriers).

Though even that authentication requirement has failed in the past, like when LocationSmart had a public demo page exploited. Inspection of the requests the page sent made it trivial to replay them with any phone number, skipping any consent checking. They just had to add "privacyConsent":"True" to the payload [5].

But yeah, it sounds like that is less of a worry now.

Instead, T-mobile is selling the location data, and basically anything whatever usage data they collect from your phone with their root-privileged app to advertising networks. They say it's a

Although their privacy page has this statement [6]:

> We do not use or share Customer Proprietary Network Information (“CPNI”) or precise location data for advertising unless you give us your express permission.

The 'express permission' here is deceptive. Users default to permit this, so it's hardly 'express'.

Further, they recently mass reset user preferences to clear the opt-out setting for users who previously opted out. Without consent.

So basically everyone is 'consenting' unless they very recently opted-out. Though I have little faith they won't change this from underneath their users again in the future. No doubt in the fine print of one of those 'annual privacy notices' or some such.

Still, if the wording and definition of 'express consent' is questionable above, they word it more explicitly in the more detailed privacy policy [7]:

> We and others may also use information about your usage, device, location, and demographics to serve you personalized ads, measure performance of those ads, and conduct analytics and reporting.

Their privacy page is deceptive about how anonymized their collection is [6]:

> When we share this information with third parties, it is not tied to your name or information that directly identifies you. Instead, we tie it to your mobile advertising identifier or another unique identifier.

Tying it to a mobile advertising id, or any kind of unique identifier, is not de-identification. It is trivial to tie this to an email or a larger profile generated by an advertising network and combine with, say, your desktop web browser. Or any account you login with that is associated to your email..

It's despicable. But sorry, I'll stop ranting now.

[1] https://www.vice.com/en/article/nepxbz/i-gave-a-bounty-hunte...

[2] https://www.nytimes.com/2018/05/10/technology/cellphone-trac...

[3] https://www.zdnet.com/article/us-cell-carriers-selling-acces...

[4] https://www.nationalheraldindia.com/international/over-dolla...

[5] https://www.robertxiao.ca/hacking/locationsmart/

[6] https://www.t-mobile.com/privacy-center/our-practices/privac...

[7] https://www.t-mobile.com/privacy-center/education-and-resour...