Wiring or microfilters ??
All telecoms devices must be microfiltered or they will interfere with the broadband signal. ALL phones, faxes, answer machines, sky box, house alarm etc. Poor quality microfilters, poor telephone extension cabling or DIY connections, loose plugs or sockets can all cause signal problems.
You might find these (long) posts relevant about testing whether it is being caused by bad telephone wiring inside the house. click here click here