It would be unusual to have a faulty RJ11 lead, unless the securing clip at either end was broken, or the ends were not plugged in until secured by the retaining clip.
What is possible is that the act of unplugging the old lead and plugging in the new one, has cleared a possible bad connection between the front filter plate, and the socket on the back section.
Time will tell if that is the case, as the fault may return.
