treenutz68 Thanks for the reading material!
I get that people may choose to layer VoIP "phone" service on top of a data-only cellular service for many reasons, including wanting to give out different numbers to different classes of contact. But my question was "Who loses the ability to location-track me if I switch to a VoIP number?". You provided a bunch of links to good journalism about location tracking, but some of that material (the piece in The Markup, also two EFF pieces) wasn't about cellular tracking, it was about authors of location-based apps (maps, weather, location-based dating, etc.) selling location data from the apps, not from a cellular network. It's not clear to me how I would benefit from switching to VoIP calls and a data-only SIM if I were running a bunch of privacy-invasive location-based apps on my phone.
With respect to a civil court subpoena for location information, I am not an attorney, but I believe such a subpoena would need to occur in the context of an existing case, including formal notice to the defendant, and that the subpoena itself would be contestable by both the defendant and any third parties. So this could happen, but it's not clear that a VoIP number would provide substantial insulation, since I believe a plaintiff attorney, with court permission, could compel a defendant to turn over any location records available to the defendant, including compelling the defendant to obtain the location records from the data-SIM provider.
Other specific examples were about law enforcement -- a protest, and a murder scene. Especially in the case of a murder scene I am skeptical that a data-only SIM would result in a police investigation just skipping that location track and moving on to the cellular tracks of everybody else. In fact, the presence of a mobile data-only SIM (especially a roaming foreign SIM) seems like it would stand out and attract special attention. I further suspect that, given a location track for a data-only SIM over any substantial portion of time, it would be feasible to de-anonymize that track to a person (there is published literature on this being easy to do).
It is true that U.S. carriers (and, for all we know, carriers in other countries) were selling customer location data with wild abandon some years ago. That was bad. Even if those carriers have generally stopped doing that, clearly there will be some leakage in some cases -- especially, perhaps in high-threat situations such as domestic partner abuse. I can see how some high-threat users might benefit from using VoIP calling to provide some (unknowable) amount of risk reduction. But I wouldn't know how to rank the importance of such a measure compared to others (moving to another town, hiring a bodyguard, etc.).
Your point about location tracks piling up over unknown amounts of time is valid. But unless a detailed location track remains anonymous, it's not clear how much it helps for it to be a data-only SIM with VoIP calls on top of it.
Overall, cellular networks are inherently tracking networks. Various legal and regulatory requirements mean that carriers must closely track devices (they may be happy to do it anyway, but regardless they are required).
Back to my question: "Who loses the ability to location-track me if I switch to a VoIP number?". Based on your examples, here's what I think I see:
- Privacy-invasive apps using GPS, or Google's Wi-Fi-based location service, would be just as good at tracking me if I were using VoIP for calls,
- Civil discovery could still compel me to provide location-track information if I were using VoIP for calls,
- Law enforcement might be mildly inconvenienced if I were using VoIP for calls, but they also might be excited and energized,
- A non-governmental attacker willing to break the law a little to track me in particular might be inconvenienced if I had a data-only SIM and used VoIP for calling.
I get that using multiple -- potentially disposable -- VoIP numbers can make my phone calls (and SMS/MMS) more private. And I can see how some people with an elevated threat level might want to reduce the risk of being tracked illegally by switching to VoIP calling (and also might want to switch cities, hire a bodyguard, install security cameras, etc.).
But I suspect for many users the extra cost and inconvenience is non-trivial, and the amount of tracking reduction is modest. As a lifestyle choice of course that's fine! But meanwhile I think many people may be overlooking the location tracking that can happen via apps.