On ADSL self install, the faceplate
into your house will be carrying signals of both frequencies,
so you need to ensure that the relevant devices are able to interpret
these signals correctly.
You should install a filter on each
telephone device that uses the analogue phone (low frequency signals).
This means all telephones, fax machines, sky boxes etc will need
to be attached to a filter.
However, your adsl modem is able to live quite happily with the
signals that is transmitted on the lower frequency.