You will need to install one ADSL filter per handset.
Some people who are sensitive to EMFs find they react to ordinary wired telephones. One of the reasons this may be, is that there can be broadband radiofrequency "noise" coming through the wire from the telephone socket. This noise is almost universal in the UK now, even if you personally do not have a broadband internet connection. This double filter reduces the electromagnetic noise dramatically, as can be seen in the graph below. Simply remove your phone from the phone socket, plug the filter into your phone socket and plug your phone into the filter, and the fields will be instantly reduced.