But not to go make a visual check on the physical state of exterior cabling, unless called to a fault.
But who said anything about going out to it, other than for a fault?
Regardless, however the "fault" was caused, if the cable had not been installed adequately (i.e. adequately protected/buried out the way of "routine" type gardening) it still would be BT's liability!
Certainly, if I was in the OP's position, I would be arguing that if BT tried to charge me!