Taiwan's new 'electronic fence' for quarantines leads wave of virus monitoring

Yes and no. Airplane mode, turning your phone off, etc.

But also, even if your phone is pinging the tower with your device’s IMEI... how would they know it’s your phone’s IMEI? There’s no registry mapping IMEIs to legal identities, in any country, AFAIK. Carriers have that information, but you wouldn’t have yet signed up with any Taiwanese carrier. So the Taiwanese government could only get that information from a foreign carrier.

(Yes, you could plug all the tower pings into a graph DB and run Dynamic Network Analysis to figure out what common unidentified IMEI matches the route pattern you extracted from the CCTV data—but you’re expecting a lot here; that hardly works even when you have tens of thousands of bits of evidence of e.g. a crime ring; and that kind of analysis is one of the “slow checks”, like DNA PCR assaying, that takes months to run—especially if you’re having to go process-of-elimination by adding everyone else’s IMEIs into the graph to find the “UFO” IMEI [which presumes that doesn’t also involve O(N) warrants somehow. I’ll be nice and assume it doesn’t.] Add the fact that the only people trained in Network Analysis will be in your country’s fraud squad, and someone between Customs Enforcement and Fraud will have to come up with the idea of getting the fraud people to use this technique outside of its usual domain of application, and... not seeing it happening.)

Basically, for the Taiwanese government to know your phone’s IMEI, you would have either had to tell them the IMEI on the customs form†; or they would have to ask your previous phone provider in your country of origin what IMEI they last saw registered to your IMSI (in turn attached to the phone number on your customs form); and then, provided your domestic provider had any reason at all to want to answer that question (they certainly wouldn’t be compelled to), then the Taiwanese govt would be able to track you.

But that helps nothing if, again, you just provided the phone number of the hotel you’re staying at as your “contact number” on the customs form, as they ask you to. Then they’d need to take another step back and figure out what your cellular numbers in regular use are, given only your other details, without even knowing what carriers you have relationships with in other countries. (Sure, okay, look it up on LinkedIn. That works for a normal person. I’m assuming a person motivated to hide here.)

It also helps nothing if you bought a new phone at any point between when you last signed up for a SIM in your last country of known residence, and when you showed up in Taiwan; and it also helps nothing if you just set your phone to spoof your IMEI. (IMEIs are like MAC addresses: a hardware-fused default with a software-programmable override!)

† ...or they could read it off the millimeter-wave scan of your phone they took at the airport, if they tune the scanner just so. That solves everything for them except the IMEI spoofing, really.