Had an interesting thing happen

DSL only requires 1 wire to work. Your phone line requires 2 wires to work.
The DSL switches back and forth so fast between the 2 wires, that it doesn't realize that one side is broke/in trouble.

At least that is the way it used to be. Don't know if it still is.

For years now, most of the phone companies don't have a filter on each phone. They have it all done in the NID outside on the back of your house. There is one filter in there that goes to all your phones and the other side has no filters that is for your DSL. Cheaper and easier for them to trouble shoot a problem even without sending a tech out to your location.