When I received a trial period for DSL internet access they gave me a bunch of these devices. The connect inline with the wall and your telephones.
My question is, as there seems to be four identical sections to the circuitry, what are they and what do they do?

Thanks

 

DSL uses higher frequencies than the POTS audio range for communications. The circuits are probably low pass filters for remove DSL interference from the audio to and from the telephone receiver.

 

The circuits are probably low pass filters for remove DSL interference from the audio to and from the telephone receiver.

Seconded. Wikipedia has an article on DSL Filters: http://en.wikipedia.org/wiki/DSL_filter

 

That's right. I had DSL. Here is an additional tidbit. I also has a very old telephone with carbon mic, etc. in the basement. It worked fine without the filter. The other phones needed it. John

 

Better use always filter for telephones. You may get less noise margin