have you tried another phone in the socket with all other phones unplugged?? if you feel confident and know which is the master socket you could try unscrewing the small faceplate at the front and bottom of the socket.be careful as you pull it off as there could be wires connected to the back of it.
when the faceplate is off you will see a port to plug your phone into,this will test the 'bare' line coming into your property from VM's network,make sure you know the phone youre going to use is a worker by testing in a neighbours/friends/familys socket first.if its still dead its VM's responsibility,if its live youve narrowed it down to either the extension wiring or the faceplate at fault.