There are phones that have software built in that use time domain to report it's relative location and report to a controller what site it is connected to and which ones are in order for hand off by signal ranking.

I read a book were medical companies had all the toilets in america wired and your "export" was analysed via a huge network. Crazy, but you never know. When a "sick" person was identified with some deadly strains of DNA virus, they were whisked away.