What happens if my telephone line or my cable is cut?

That makes the supervision matter more than the path itself. A classic telephone line is cut from the street and says nothing. A network connection goes down with the router or with the neighbour's power cut. A mobile network path can be jammed. The difference between a serious system and one that only looks like it is whether somebody is checking every few moments that the path is still open, and how often — the alarm transmission standard classifies exactly that, and it is a figure you can ask for in writing.

And there is a physical part that gets forgotten: the panel and the communications unit go inside the protected area, never in the entrance hall or in a cabinet you can reach from outside, and with their own tamper protection. An alarm panel you can get to before the entry time runs out is the oldest design fault there is and it still turns up.