No you don't - digital voice can be delivered perfectly well over broadband carried over existing copper cables.
BT's own publicity for DV has, I think, caused this confusion because it talks about how DV will "retire old fashioned copper cables which are becoming obsolete".
A very large number of houses are likely to stay on fibre to the cabinet, with copper from the cabinet to the house, for a significant time and no amount of changing from analogue phones to DV will change this. BT is retiring the analogue phone equipment at the exchanges, NOT the copper cables!
