Thanks ... To emphasize a point that you seem to imply, the goal of security is to raise the costs of the attacker; anything can be defeated, of course.
If by "two problems" you mean that VOIP adds an additional problem, I don't quite grok it. It isn't a panacea, as you point out, but seems like a clear improvement.
Another advantage of VOIP is that you can easily obtain throwaway phone numbers.
> If you're making VOIP calls over a device that is itself connected to mobile networks ... you've still got the connectivity of the device itself to track. Presumably that's a long-lived relationship. At this point the information is limited to location data, but that, at the postal-code level is again sufficient to identify 90% of individuals within the US, based largely on residential and workplace locations.
Good point. They still don't know who I talk to and when, but they certainly can figure out who I am. I wonder how expensive the latter is, which I'd guess it depends on whether that analysis and the sharing of it is done automatically or takes a special request.
> The tether is connected over WiFi
I'm not sure that helps privacy: Wifi networks are likely shorter range than 5G cells, and the networks are well mapped. I suppose it does require involvement of someone with the map, but that might be easy to obtain.
> the MAC address space is already repetitive, and MAC addresses can be arbitrarily changed at the OS kernel level
I think iOS and Android randomize MAC addresses these days ?
> Burner phones on a one-use / short-use cycle would probably be preferable.
Yes, but a single burner phone, between the hardware and a one month plan, can cost $75-100. Using lots of them is out of reach for many people.