This is right on the line of creepy surveillance and interesting public art project. I kind of like the idea but not a fan of the fact that it’s recording in stealth. I wish it were more transparent about it. Most people wouldn’t care anyway and it removes some of the discomfort of listening in on a bunch of strangers.
Tmobile does it as a service but it’s a paid one and inconsistent in accuracy. I had it for free as part of a plan for a while and told them no thanks to an additional charge. Not sure it’s worth paying $5/month.