The Telco has complex troubleshooting equipment to determine if some badly behaving device is connected (usually faulty phone equipment) They will first likely send somone out to troubleshoot the lines.
Mostly they just test for grounds on either tip or ring wire, load impedenace, ring current draw and signal level allowance. None are hard to stay in compliance with if you know what you are doing or at least research the requirements.
So lets say you dont allow them to troubleshoot. I dont know what they will do. They do have an obligation to keep telephone service running properly for 911 emergency reasons. I suppose their options will be shut your service off altogether or go get a warrant or both.
You could probably play ignorant with them and get away with it. Better still if your device doesnt bother the line, they will likely not detect it. But just keep in mind that people have gotten into trouble over it.
1) Fooling with phone system with non approved / faulty devices
Yes, didn't Steve Wozniak of Apple computer get his start building and selling 'Blue Boxes' for obtaining free LD calls
2) Trying to steal power from high voltage lines with donut transformers
The price of power would have to get a whole lot higher for me to try such antics, besides out power is underground and I'm too old to be digging trenches
3) Trying to jam / blank out a frequency band regulated by FCC
No challenge, I've a HAM license that I would rather not lose
People try these things all the time and you CAN get into real trouble if caught.