Take the faceplate off the Master Socket and check that the extension wires are connected. You should have the Blue & White wire(s) to connector 2 and White & Blue to connector 5. NO OTHER wires need be connected.
Having said that they may have been disconnected deliberately because they were causing a fault.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix corded phones.