For realizing with certainty, it was not very easy, because I had to be comparable with filters that work for sure. I bought all three different filters in a DIY store to compare them. The 3 work perfectly, proving that the two copies of "Lindy 75109 ADSL filter-separated 2 taken RJ11- White" are defective.
I had read the reviews for many ADSL filters on Amazon, and apparently no filter is reliable. :(