My own house has four DECTs. They've all been bought at different times, so each one sits on an active base station, even though it's only purpose is to charge the handset, so there are 8 sources of DECT yet, hard as I try, I've never identified any degradation to ADSL caused by these phones.
However, playing Devil's advocate to my own observation, DECT uses GHz frequencies, but it is pulsed at (I think) 100Hz. When I hold a handset close to an AM radio, I do hear interference, so I'm open minded to the possibility that it may in some circumstances, if placed close to the router, have an effect on ADSL too.
To put it in context though, GSM mobile handsets are similar - GHz frequencies, pulsed, but with a much stronger radiated power than DECT. It could be argued that any attempts to avoid DECT completely are relatively pointless, unless you also ban GSM mobiles from your home, and from the homes of any nearby neighbours, passing cars etc...