Private services that can sell your location to pretty much whomever with only your phone number.
Try the demo, it's insanely creepy. I lock things down pretty well, but it seems this might be coming from the carriers themselves.
This makes me want to get a dumb phone (Nokia 3310 anyone?) that I judiciously power down when I'm not using it.
@codydh Not working for numbers outside the US, it seems. But still, this is insane.
If you really need to use a SIM card, here's a recommendation: Get a Samsung Galaxy S2 (50-80 bucks used), install Replicant, and use airplane mode frequently. The advantage over some old dumb phone is that you at least have a fully free OS and you can use properly encrypted messaging (e.g. Conversations).
Or get a really dumb phone, but only ever switch it on when you need it (this might be tough)...
@codydh I completely agree. The situation is very frustrating. Unfortunately, WiFi and NFC tracking will be the next big thing for sure. They'll track your MAC address whenever you log into a public WiFi and sell your profile to others. Actually even if you don't log in. Or they'll display targeted ads inside a store when you move towards a certain area. Creepy cyberpunk shit.
So, not sure if WiFi is the solution in a highly commercial environment. It's all shit.