I have posted keys and sounders and such which connect to outside lines. So does the ordinary telephone and the high tech DSL modems. Now imagine that you connect something in your house to a multi-mile long antenna to pick up lightning and AC disturbances. We you have if you connected any of the above to a telephone/telegraph line.
Your first line of defense is the primary protector. It sits outside the building where the line comes in. Today they are high tech. But they used to be just two carbon blocks placed 0.003 inches apart. The traditional 3 mil carbon block. Many safety standards still assume no more than a 3 mil carbon block. Basically, the carbon block worked by having the surge arc across the 3 mil gap. Now days the lines are protected by gas tubes or semiconductor devices. So what does the 3 mil carbon block protector look like?
Remove from our house many years ago: