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Spy on your family member's movements for just 10 Dollars a month! Satisfaction Guaranteed or your money back!
(sh.itjust.works)
This is a carrier in the USA (T-Mobile).
I did a quick search for the other 2 carriers using the term "[Carrier Name] Family Tracking" and Verizon and AT&T also seems to have it.
And according to https://www.t-mobile.com/support/plans-features/t-mobile-familywhere-app, it says:
FamilyWhere uses geolocation data from the T-Mobile network and is not affected by changes to device location settings.
So it appears that its using cell tower triangulation. Turning on Airplane Mode should stop it (assuming there isn't a separate tracking app on your phone)
Oh Wow, What a wonderful tool for abusive spouses and abusive parents. And telecom companies are making money off of it. ๐
TLDR: Its a good idea to get your own separate cellular plan.
Verizon has had this since flip phone days.
As you noted, it uses cell tower data, so anything that has a SIM can be located pretty accurately. (Technically anything with a cell radio, as towers will track that).
It's how they had mapping on flip phones.
I remember talking to peers about location tracking with the first digital cell phones circa 1996. We were concerned then, but couldn't get any non-tech folks to hear us.