After some days waiting i got confirmation of the helpdesk that i was able to upgrade my ADSL modem. (The royal dutch telephone company wrote that an update could only be done with the previous bios instead of the older one i have..... they were wrong.)
Took about 30 mins (the most was the making of screendumps to get a backup of the settings..)
Seems to be working ok.
The voip only worked if you go to the XS4all support site and run a modem-voip-setup script.
But now it works ok.
I now have 2 voip numbers and working nicely