What is this device used for?

Hello,

It's indeed on your phone line...

So,
There are two types of protection:
Simple with fuses
Fuse plus surge protector

Fuse protections are not removable; it's a simple support to which two fuse holders are connected, see photo here:
http://farm2.static.flickr.com/1395/1468...

In this case, just remove the fuses and test them with an ohmmeter.

In the case of a surge protection box, it is a box with a clipped cover; with a small screwdriver, you need to pry open the clips to simply open the box, and you can test the fuses in the same way. The surge protector does not need to be tested; it is non-conductive (isolated).
see here:
http://www.connexion-internet.com/astuce...

WARNING! If you use this line for internet (ADSL connection), you need to remove the fuse box or surge protection box. Indeed, this type of box (which was installed long before ADSL arrived) often causes connection stability problems. Removing it doesn’t pose any issues since line protections have long been installed in the central office's distribution frame, so this subscriber-side protection no longer makes sense.

In this case, you need to remove the surge fuse box and replace it with a simple terminal junction box like this one:

http://static.connexion-internet.com/images/cablage-boitier-12plots_2_www.connexion-internet.com.jpg

On which you connect the two incoming wires from the protection box with the two outgoing wires

(info taken from a forum)

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