Keep in mind too, that sometimes a text message will go through when a cell call will not.
I use the "Find my Friends" app (with my two sons and wife) so we can find each other when we need to. This requires the phone to be on and to have cell coverage, but it gives a pretty good idea as to where they are.
An iPhone also has a "Last Known Location" feature.
To enable this, go to Settings --> iCloud --> Find My iPhone --> Send Last Location - turn this slider to on.
When the battery gets to a critically low level, the location of the phone will be sent to your iCloud account, where the information can then be retrieved and used as a starting point in the search if needed.