And supplementing TheEulerID's comments, check your phone line, both outside from the exchange and inside over extensions etc, is "clean" by doing the Quiet Line Tests, before going any further.
For VDSL, the final part of the line from the PCP (D-Side) and within your house are particularly important.
Also try to get an accurate idea of the actual wire length of that D-side, as that also can affect the performance of VDSL.
Check this thread-
Edited by eckiedoo (Sun 06-Sep-15 07:54:17)