We've been mucking around with ADSL2 lately and it has proved to be very frustrating. The problem is trying to locate the issues. Firstly, you change all the equipment at your end. Next you upgrade everything at your end. After that you blame the ISP and give their tech support a hard time and just when you think you are going to pull all your hair out you replace the ADSL filter on the phone line and viola, everything works.
From what we can work out ADSL, operating at higher frequencies, is much more sensitive to interference than normal ADSL. So using an older ADSL filter on an ADSL2 line may not provide enough “filtering” to allow adequate reliability. If you are having issues, the first thing that we would recommend you try is buying and installing a new line filter. It worked for us.
We have also seen some cases where ADSL filters have caused issues on lines so for what it is worth go out and buy a few filters and keep 'em handy and if problems develop try a new filter first. It may save you the hours that we wasted.