Most RJ11 modem cables are flat ribbon cables. No twist.
Good RJ11 cables are of twisted pair construction.
Openreach go to a lot of trouble to guarantee the twist in the cable supplying your master socket.
Your ISP who generally does not give two hoots about anything saves 50p. Hey presto ribbon cable.
Twisted are hard to find. You can make your own if you want.
As for length it's a 'How long is a piece of string and does its colour matter?'
Depending on the frequency of the interference, called noise in my world, and more importantly its harmonics.
This will determine if a longer or shorter cable will make things better.
The answer to will a shorter cable make things better is maybe.
A dirty great big long one maybe.
As the noise is out of, I hope, your control and unique in every geographic location it's a lottery.
Right angles good, parallel bad.
CAT5/6/6e length over RJ11 length? CATx I'd bet my house on it. If I had one.
Digger of Long Shallow Holes. Trenches not graves for myths. But then again.