Some other posters mentioned that an unused land phone line may still offer dial tone to provide for emergency 911 service. Is this a recent offering?
The phone boxes within my apt have a forest of wires, it appears about six pairs. It's been a while since I tested them, but other than the ones I use the lines are dead. They may not be physically connected within the master jct box outside.
Cell phones purchased at a flea market have that capability, I believe that was mandated by law a few years ago. I keep such a phone in my car glove box (with an power adapter) in case of emergency since I don't carry my real cell with me that often.