The discovery was made by a TV crew member looking for filming locations in the Wye Valley area of outstanding natural beauty (AONB) in Monmouthshire.
The 2015 directories were found with other fly-tipped rubbish in the Livox quarry between Chepstow and Tintern.
BT said the directories may have been dropped off by contractors for delivery but taken by unknown people before local distributors arrived.
A spokesman said the relevant division would be informed.
The land agent for the site has also been told.
Crimestoppers said statistics showed there were 31,000 incidents of fly-tipping in Wales in 2014-15, with 80% of incidents involving household waste and clear-up costs estimated at Â£1.8m.
Andrew Murphy from the charity said: "The cost of clearing up is only one part of the problem.
"Fly-tipping is a blight on our communities. It poses a threat to humans and wildlife, it damages our environment and it spoils our beautiful towns and countryside."
A large number of BT phone directories have been dumped at a beauty spot.