Should the fax machine sit behind a DSL filter or not?
Yes it should.
ADSL Filters & Splitters
Generally speaking the terms "micro-filter", "splitter" and "ADSL
filter" mean the same thing.
When your telephone line is ADSL enabled it is able to carry
information on different frequency ranges: ADSL uses the higher
frequencies to transmit data, whilst voice uses the lower frequency
Although you will still be able to use your telephone as normal once
your line is activated, the transmission of adsl data on the line will
In order that the adsl data does not interfere with the normal voice
data (and vice-versa), you need to ensure you install a filter on each
device that uses the lower frequencies. These devices are telephones,
fax machines, answer phones, sky boxes.
Source ADSL Filters & Splitters
Why do I need ADSL filters (microfilters) and where do they go?
A microfilter is a device that allows broadband to work at the same
time as your telephone service. Without microfilters you may get
problems with your BT broadband or experience noise on your telephone
Normally, you'll need a microfilter for every telephone socket in your
home where you've got some kind of telephony or broadband equipment
plugged in. This equipment includes:
- Your BT Home Hub, or another type of broadband router or modem
- Answering machines
- Digital TV boxes (for example, Sky boxes)
- Fax machines
- Alarm systems
Source Why do I need ADSL filters (microfilters) and where do they go?