Well first of all, if their service critically depends on wireless communication, then that's bad design.

Further, I imagine you turn up the amount of power of the jammer until the person on the phone gets bad reception.

I don’t know what you mean by critically. But everything from calling law enforcement for pedestrian and vehicle strikes to handling special operations (i.e. single tracking in a work zone) uses wireless communication.

There are also defect detection systems that rely on radio transmissions to indicate if a train is for example dragging equipment.