Given that it is technically feasible, I'd assume it's possible for a mobile phone base station to geofence something like an embassy, or park bench, and have a

Given that it is technically feasible, I'd assume it's possible for a mobile phone base station to geofence something like an embassy, or park bench, and have all mobile phones entering the geofenced perimeter go into surveillance mode, and return to normal on exiting the perimeter. Given that phones have a precise GPS locked timebase, I'd even assume that it is possible to beamform the signal from multiple phones, allowing conversations to be picked out from background noise. Continuous monitoring, as long as there is a continuous stream of mobile phones passing by.