The jammed telephone wire is approx 5mm dia. Both ends are accessible and a v. small amount of movement at one end can be felt if pulling the other pretty hard - probably just the plastic stretching. I don't know how long a length is jammed and where the jam is.
I am happy to sacrifice the cable if I could remove it 'shredded' but I would then need to clear debris out of the channel and open it up to allow a new cable to be pushed through.
Is there such a thing as a flexible, narrow (2-3mm dia?), abrasive wire that I could thread down the hole, fix one end into an electric drill chuck and 'thrash it about' in the hole to make it bigger. That is assuming I could get enough of the old tel cable out in the first place!

An electricians cable snake... fish tape... would work well
It is flat and thin... like a dipstick on a car .It is spring steel,pretty tough
You didn't mention how long in feet your project is

Thanks Joey,
The jam is between the tel socket and the floor - a distance of about 12", but the actual length of the jam is probably less than this.
Cable snakes look as if they're too long, may not be abrasive enough to open up the hole in the plaster or mortar - even if they can be inserted in the first place. The cable diameter is only about 5mm.
What I need (ideally) is a long, flexible (!) masonry drill about 7-8mm dia, but such a thing doesn't exist