The only reason you need a filter, is because when data is traveling along your telephone line, it causes noise in the handset. Indeed, if NO data is flowing, you can hold a perfectly normal telephone conversation on a ADSL enabled line.
So, if you have a cordless telephone, the telephone analog signal is arriving at the base station, which should be filtered in the normal manner. Once it is re-transmitted to the handset, it requires no further (re)-filtering, as ADSL data is no longer an issue.
Hope that makes sense!