when installing a phone line in a house it is (legal) and common practice for BT or bell cable or NTL or C & W (you get the picture!!) to utilise any or all, existing wiring AFTER another companies master socket,specifically any extentions that have been fitted by prior companies,as these are considered 'customer responsibility' after 12 months of being installed.
that is,any faults on these wires/sockets are deemed payable by the customer if they need to be fixed/replaced.
now im not saying it doesnt happen but you arent allowed to remove another companies 'hard wire' from a socket to install another companies 'hard wire'.so (good luck!!) but i think your beef must be with BT