The phone companies already do this, more or less, as is shown in court cases where cell phone records are brought in as evidence
A decade ago that data was a little more iffy (i.e. it was more a good estimate (typically within half a mile or less) than a true location), but with a combination of more towers (and therefore more data points), the ubiquity of smartphones (which check in more often, are doing geolocation related things, etc), and better / more accessible/well-known analytics tools, is think even 6 months would be a generous time-frame
A decade ago that data was a little more iffy (i.e. it was more a good estimate (typically within half a mile or less) than a true location), but with a combination of more towers (and therefore more data points), the ubiquity of smartphones (which check in more often, are doing geolocation related things, etc), and better / more accessible/well-known analytics tools, is think even 6 months would be a generous time-frame