T&T: Anchor alarm/cell phone

[Faure, Marin]

How does a cell phone function as an anchor alarm?   I assume we're
talking a phone with GPS capability, but how do you use it as an
anchoring alarm?  My wife needs to get a new cell phone, and if using it
as a position alarm actually works reliably and without too many false
alarms, I'd rather leave a cell phone on all night on its own battery
than run our big plotters on the boat's batteries all night.

______________________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington