What about the hundreds of thousands or millions [0] of illegal border crossings every year at the mexican border though?
[0] Naturally good stats for this are impossible to find, but in FY19 851,508 individuals were apprehended at the american southwest border [1]. It seems reasonable to assume the same order of magnitude of people successfully crossed.
> Sky News understands the [UK] Government has asked mobile network O2 to hand over anonymous mobile phone location data in order to ensure people are following social distancing guidelines during the COVID-19 outbreak
I suspect it's at least being discussed.
(I'm also skeptical that stopping illegal immigrants will be as simple as tracking phones, but whatever)