I bet it looks better in their books to have 40K paying customers more than not having those, so they just ignore it as long as not causing more problems than they are worth it. My guess is that the telcos were making half a million euros a month from these.
Can they know the SIM location precisely? I believe they can only triangulate multiple towers to determine a radius. If they could pinpoint a specific, narrow location, it'd be easier to spot unusual concentration.
Depends. They found one of these in New York but it’s very easy for 10s of thousands onto gather in a relatively small area. For example, New Year’s Eve, sports/concert at msg, regular foot traffic at Times Square, etc. so I think barring even antennas shenanigans, disguising it could be not impossible.
(I also understand they rarely use all active SIMs at the same time but instead rotate through in order to avoid arousing suspicion)
> but instead rotate through in order to avoid arousing suspicion
Also to use fewer resources! Compared to the SIM cards, the radio/modems are expensive. It's cheaper to reuse one radio/modem cycling through different SIM cards for just long enough to receive pending SMSes. But not too many, or you increase latency. It's probably more suspicious to have the same modems cycling through SIM cards than to have all of them always connected.
If they mostly received inbound traffic, the carrier actually gets paid for it, so may not have any incentive to stop this. Carriers generally only care when SIM farms place outgoing traffic (it allows them to use cheap/free consumer plans instead of expensive SMS providers).
It's not impossible that they have directional antennas pointed at different towers nearby-ish, if you do directional antennas the triangulation thing kinda fails.
Just speculation though, it's more likely they just paid the right people off.