Not all RJ11 cables are equal, some are "straight" & some are "crossover". Try looking at the colours of the cables under a magnifying glass or a zoomed phone image. Do they follow the same pattern at each end, or do the wire cross from outer pair at one end to middle pair at the other? Whichever you have, try sourcing the other.
Can't guarantee that's the issue but it's what I'd try first, assuming that you know the socket you're connecting to is good.
