The overwhelming majority of customers served from telegraph poles are ( obviously ) fed overhead , if your service is delivered underground it could be from the telegraph pole , or could be a distribution point in the ground in a jointbox for example , if it is the pole with the cable running down the pole to ground level then underground towards your home, in either case a cable should either inside a duct or be an armoured cable, and there should be a point on your external wall where the cable or duct or both appear, and an external box on the wall where another ‘internal’ cable runs inside the property to the position of the master socket…it’s the block on the external wall you need to locate , as this is the only point initially will be known .
TBH, if you are getting work done , then the builder/developer/contractor you would image has had this situation many times and would know what to look for and how to avoid any underground services, and probably have their own ‘cat and geny’ type device to track underground cables ….it’s unlikely that OR will have a particularly accurate record of the route of the cable , and although they offer to locate their services in public areas ( because damage here is likely to affect many’s customers ) , if you want the cable tracking in your land it could be charged for…..
Its Openreach you would need to contact about this , not BT , that is assuming BT are your provider.
