I use my wife's old iPhone 5s. Got a 40gig 4g data only plan with Telstra that expires every 365 days.
It's hidden in our van and always on. It uses the vans battery that gets charged with the solar panel.
I have the Find My IPhone app as well as a GPS tracker app that records movement every 10 minutes.
It uses about .5 of a MB of data per day.
We also use it as a mobile Wifi hotspot when we use the van so we can stay connected without using our phone data.
Also used to stream music and is bluetoothed into the Vans sterio.
Our van is out the front of our house.
Security is:
Towball lock
Wheel lock
Cable chain to the light pole
And
Security cameras from our house can see either side of the van.
I think it's pretty safe but it's not impossible to take. Gotta love portable angle grinders now days.