Those are "protectors;" they are (internally) a gas popper type device. They "protect" from the external surges on incoming lines, and "protect" the company from customers putting damaging voltage on the line upstream.
In order to work, there needs to exist a qualified ground connection of ~25 Ohms or less (less is better). Over 25 Ohms, and the primary protection (those poppers) don't fire soon enough and damaging potential can enter the prem.