Well, this cable is used to connect my analog phone with the FON1 input the Fritz! Box. Before a TAE plug at the bottom of the box (FON1) had been inserted. The exchange took place more or less for optical reasons. Thus I was able to move this RJ11 together with the RJ45 cable. Now looks much tidier than before. Voice transmission quality and properly. By 3m length one can vary the location of the phone also.
Conclusion: Quality does not have to be expensive!