We live part time in rural Wisconsin. We have three "spy phones". They have a sensor on them and if someone walks past it, it will call you or the number you put into the memory. A speaker stays on for 30 to 60 seconds so that you can hear what is going on.
Last month we got a call and could hear footsteps. We called the sheriff's office and they sent 3 squads out only to discover our neighbors (who have a key) removing a downed tree. Our neighbors forgot to call us and tell us that they would be working in the yard.
At least we know it works. Very cheap. Phones are about $60.00.