The whole mobile phone triangulation business is very hit and miss. We use it quite often ( well, the Police do) in my MR and SAR work. It's a great way of finding out if Person X is vaguely in the area you are searching, or if they are in fact 200 miles away in Berkshire, but don't expect it to produce an accurate 8 figure grid reference...
The region I operate in is currently trialling an app where we text a code to the missing person, and if it's a smartphone they can click on the embedded link and it emails their location to us using the inbuilt GPS in their phone. Working very well so far!