Bolo, I spent my younger years as a BT engineer and can tell you that reversing the polarity will make no difference at all,this used to be a problem years ago but all up to date phones,systems and modems will adapt to any reversal,first of all are the connections in properly,are they screw in or a krone connection because if it is a krone connection and they havent been punched in correctly it could be a high resistance dis,if it is as you say a master socket it should have a capacitor in it and even if its a slave socket you will get dial tone but incoming calls will not ring. Try to connect the terminations again if still getting no dial tone I suspect the socket is faulty. If you ask BT to fit this they will not touch any non BT sockets or wiring, also choc blocks are a non starter or as engineers call it star wiring as this will affect the dsl signal,all extensions should run from the master socket then to the first extension,second,ect,ect