Related: There are APIs out there which enable you to write an app to allow your users to accept such pings. It costs quite a lot per ping, however, and if i re

Related: There are APIs out there which enable you to write an app to allow your users to accept such pings. It costs quite a lot per ping, however, and if i recall may have to be allowed by SMS for each one by the user. It was seriously looked at before the iphone and pre ubiquitous GPS. The triangulation was done on (Vodafone?) proprietary boxes.