there used to be an app called 'life 360' where you can track a smartphone with a smartphone. also I think you can buy a cheap chinese tracking device on eBay, you send it a text and it texts you back it's coordinates.
hi I have a mini charger that is wired in to permanent 12vdc supply this gives out about 5vdc which keeps the phone charged solar panel charger keeps leisure battery topped up, I just used a giffgaff sim £10 quid every few months, or a least i would if I could find a decent app to use