How to connect 56K modem to ADSL line ?

I have a 56 K modem that I want to use as a backup if my ADSL service goes
down. The idea is that this would call a dialup number, either with my ADSL
ISP or maybe a different one. My question is: should I connect this modem
directly to the ADSL phone line or must it be connected through a filter
(like a telephone) ? I guess it should be connected through a filter but do
ADSL filters have a cut-off that is high enough to allow conventional analog
modem signals to pass back and forth without attenuating them too much ?

Any advice would be appreciated




A.Robinson