Comment by Intralexical

Related discussion:

Location tracking of phones is out of control (arstechnica.com)

https://news.ycombinator.com/item?id=41930818

Related comment:

  486sx33 8 hours ago | next [–]
  About 2 years ago, an isp we use for one of our operations in Canada called R… which is also a media company and an advertising company… came to us and said hey! We have this amazing new technology , all you do is geofence your competitors and then we will retarget anyone who visits their location with your web ads for as long as you want! Since they are also the isp for mobile data , they just force replaced ads for the targets web browser. (Basically they inject ads)
  They also made it clear their system is not at all dependent on your phone location services or even your advertiser ID, since they are the isp and the cell provider they just use your SIM ESN to track you. ( cell towers know where their users are, with better accuracy than ever now )
  It worked, but it’s darn scary. This has been around for awhile.