ADSL uses high frequency to communicate back to the exchange via your telephone line. You need to put an ADSL filter on each telephone that shares the ADSL line. Without a filter, you will hear the high frequency when talking, and when you are on the phone your internet connection will be effected.
Telephone RJ11 Line ADSL Modem Micro Filter Splitter $0.99
White ADSL Broadband Modem Phone Line Filter Splitter $2.79