Your ADSL has worked fine for many years. You introduce a DECT phone and have problems with the DECT phone's compatibility with ADSL. The DECT phone shouldn't show these problems - generally it's recommended to keep the DECT well away from the WiFi Router or PC with WiFi connection to stop interference with the WiFi. I've used DECT along with ADSL for years, so have many people on this forum. I suggest you return the DECT phone and buy another make - you should not have such problems.