Working with the [Taiwan's] five major telecoms companies, the system tracks the quarantined individual by triangulating the location of the their phone relative to nearby cell towers. “Each telecom company has a different way of calculating the location of the phone, including how far it is from the cell reception tower and its direction in relation to the tower,” said Jyan Hong-wei, director general of Taiwan’s cybersecurity department. “As long as the phone is turned on, we can figure out the location. And if the phone is turned off, we’ll know that, and we can send a message to front line administrative or police officers so they can follow up.”
https://qz.com/1825997/taiwan-phone-tracking-system-monitors...
Exactly: Pervasive tracking is already available to the telcos and is being used in at least one country.