A common problem that we encounter is when internal grade cables which are only intended to used inside the home or office are run on the outside of a building, particularly white cables.
Over a period of time the ultra-violet rays from the sun can seriously degrade cables, as can be seen in the image above. The outer sheath then becomes brittle and falls away allowing rain and moisture into the inside of the cable. This eventually results in the conductors shorting out resulting in loss of phone service.
At the very least external cables should have a black sheath rather than white, but preferably external double sheathed cable should be used externally such as CW1412 telephone cable which is UV resistant.
Brown CW1412 cable 
White CW1412 cable
