Can I use a twisted-pair cable for ADSL and phone?
After some testing I finally found the answer myself.

I connected the cable's shielding to ground and I connected the wires this way:

(phone cable) - (cat6 cable)
  White ADSL  - Blue
   Gray ADSL  - WhiteBlue
  White Phone - Orange
   Gray Phone - WhiteOrange


(colors are for french phone cables, they are different in other countries)

I soldered they directly. Maybe it would be better to use a RJ45 connector.

Now my modem is synchronized at 8,5 Mb/s. Still some CRC errors, but it's not a big problem since I can get something like 7,1Mb/s when downloading.