I have been casting for 43 years. Cable sheathing is pure lead with an
insignificant amount of other elements. As such, it is too soft to use for cast bullets. Nevertheless, all lead except a few alloys that contain certain elements that render it useless for casting (like battery lead containing calcium, wheel weights containing zinc, etc.), can be used, with alloying, for casting...I would definitely add some wheel weights and up to 2%(maximum, higher is wasteful), tin to the cable sheathing and use it.
This forum is not the best venue for what you are looking for. The following sites are will likely serve you better.
http://castboolits.gunloads.com/
http://www.castbulletassoc.org/index.shtml